There was a long discussion on this some time ago, and I believe someone 
found the beer in Maryland.
Try  going to  www.dejanews.com and punching in Old Duselldorf, you will 
probably get what you need.
Peter
The following was gleened from http://www.dejanews.com  "power search"
using "Magnum AND Beer AND Romas" as keywords...
                                                                   
Date:    1996/02/10
Forum:   alt.tv.magnum-pi 
     Well, the beer he *preferred* to have (when he could get it)
was called "Old Dusseldorf".  He described it in "No More Mr. 
Nice Guy" as "...an old German beer brewed in Maryland."  In that
same ep. and in others he complained about how hard it was to
find Old Dusseldorf in the Islands.
In reality, Old Dusseldorf does not exist (yet).  It was 
invented for the show, a fictional German-style brew made in
Maryland (we can presume near Annapolis and the USNA where
Thomas spent his college years and may have consumated his love
for beer--like so many others!).  In episodes where he is
supposedly drinking Old Dusseldorf you'll notice that they
never really show us the label clearly.  But it appears to be
an invented label (not hard to do, I'm a brewer myself).
When he's drinking a beer in most scenes where it isn't stated
as Old Dusseldorf and where you can actually make out the label,
you'll notice a striking resemblance to the label on regular
Coors beer.  The layout, color, and design is exactly the same
as Coors' only Magnum's will say "Coops".  He even carries a
whole case of "Coops" (even the box is labeled as such) in 
"Tigers Fan".  I read somewhere once that Selleck actually owns
an interest in the Coors Brewing Co.  I'm not sure why they'd
go to the trouble of "doctoring" up a Coors-like label for the
show.  Why not use the real thing?  Maybe they didn't want to
have to worry about legality issues.
Back to Old Dusseldorf, there is no such thing brewed 
commercially.  The closest thing in name brewed here in the
States is a beer called Dusseldorfer which, if I remember right,
is made in Pennsylvania somewhere--maybe Jersey (I'm fuzzy on
the location.  I've had the beer and kept one bottle for my
beer bottle collection--which as of today includes 600+ 
different bottles from breweries all over the world.)  One time
years ago before I got into brewing and writing beer criticism
(and knew otherwise) I spent a whole day on the phone to MD.
I talked with every beer distributer and brewery in the state
asking if they had ever heard of a beer called Old Dusseldorf.
The responses ranged from laughter, to "Only on TV, kid", to
"Hey, fellas, we got another Magnum fanatic on the line!"
There is, of course, the city of Dusseldorf in Germany.  And in
that city are a few family owned craft-brew houses which 
produce a particular, distinct beer known to that city alone
(not uncommon to various German, and other central-European,
cities).  The Dussedorf style is known as Alt Bier ("old beer")
and is a heady, potent, aggressive beer genre which can be
an acquired taste to even the most adventurous and accomplished
beer hunters.  With the recent explosion over the past ten to 
fifteen years in the US micro-brewery and brew-pub market, some
American craft-brewers have attempted to replicate this
Dusseldorfer Alt style beer.  The American product I mentioned
above "Dusseldorfer", while a tasty little beer in its own right,
is a far cry from authentic German Alt.  One of the most
successful replications in America comes from the August Schell
Brewery and is called Schmaltz's Alt.  But there are other good
ones too.
Incidently, for your trivia pleasure, Magnum opened (ie: started
drinking) 119 bottles of beer over the course of the entire MPI 
series.  I did actually count them all!  And, yes, I know that's
scary.  :-)
Best,
David Romas
Magnum Memorabilia
(MPI's nonprofit foundation for collecting,
production, scholarship, and fan services)
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
"I'd like a beer if I could get one."  -TS Magnum "Limbo"
"A pint of stout.  I'd like a pint of stout."  -TS Magnum
                                "Infinity and Jelly Doughnuts"
--
I order a case every 6 mo. for my Magnum parties...
Enjoy!
Shawn Buchanan Greene
:-)  My significant other asked me that same question just this past weekend
at some point in my marathon episode copying session (upwards of thirty
episodes in a row)...part of my brilliant plan to get all caught up with
two months worth of requests that had been piling up on me.  (I don't 
think she was thrilled with my weekend plan: every fifty minutes for 
upwards of 25 hours I had to be in the house to switch tapes in the VCRs.)
Anyway, at one point, only half interested and probably half exasperated,
she asked me this very question.
I have no idea.  Some of my favorites I'm sure I've watched upwards of 100
times.  Perhaps even quite a bit more than that.  It's hard to say.  Some
of the ones which I don't care for as much I might've viewed fewer than 50
times.  Perhaps even less than that.  This is not counting how many times
I've *copied* many of them, since most of the time when I'm copying I
don't sit and watch the whole show.  I go off and do other stuff, like
photocopying things to go along with people's tapes, or switching tapes
on the audio deck where I might have a copy of the Magnum Soundtrack 
running at the same time, or posting to this newsgroup, or writing letters
to other fans, etc.  I'm sure that some of my episodes have been copied
upwards of 500 times; like my copies of "Memories Are Forever," "Did You 
See the Sunrise?," "Home from the Sea," and "Limbo/Infinity" all of 
which are perennial favorites amongst the rest of the fan following.
>Do you keep a list of things to look for, such as TM opening beers, for 
>your next viewing?   Surely you didn't rewatch each ep. just to count 
>that event!
Very true.  Here's what I do:  I've had the entire series on tape for
approx. ten years.  In that time, there have been several occasions where 
I've actually planned to watch the entire series, in original airing order, 
in a relatively short period of time (say, in my spare time over three 
months or so).  And part of the planning of one of these campaigns is to
generate a list of anything I can think of at the time which I might want
to watch for.  They'll be things that relate to MPI projects of different 
types that I might have in mind for the near future.  For example, in 1988 
I did it watching for:
- all of the songs used in the series (I had the notion that I might make
a soundtrack for MPI)
- all of the costuming and props used in the series (my lists made from 
that have been invaluable over the years in my own and other fans' 
collecting efforts)
- all of Magnum's notes to himself about what he'd include in a book
about how to be a world-class private investigator (I had the notion that 
I'd edit it all together -- which I did a couple years later)
- and a long list of other things that don't jump to mind at this moment.
On a couple of the occasions I had a good friend or another near at hand 
who also was really into MPI and wanted to watch the whole series along 
with me and would pick his/her own things to look for.  In '88 my friend 
John Hayes sat with me and had an inordinate amount of fun helping to 
document all of Higgins' "Oh My God!"'s and Magnum's "Hi, Guys!"'s which 
I later compiled together as audio montages for the Soundtrack.  In '91 
or '92 when I did a run through the series mainly in preparation for my
masters thesis on MPI, my friend Nic Zimmer watched with me and counted 
all of the times Magnum pauses and looks directly into the camera.  
Sometimes TM'll say something, but usually he just makes a face in reaction
to something that's just happened in the episode.  We called them "camera
looks."  A couple of times we even caught Selleck in camera looks (more
like "glances") that clearly were accidental.  :-)
I've kept detailed notes on numerous points of interest within each episode
generated from several different whole-series run-through's.  Many of these
notes went right into the Online MPI Episode Guide, for which I supplied the
"fans' notes" that follow each episode's brief plot synopsis...
http://magnum.st-wendel.de/magnum.html
and what didn't go there is random notations to myself, stuff that 
continually is useful in fanfic stories, research papers, convention
presentations, various A/V productions such as the Roger Mosley interview,
most of what I've written or supplied to most of the MPI-related websites 
that exist, this group and rec.arts.tv, fielding (rigorously) the questions
I receive daily from all parts on any aspect of the series I ever could
imagine, etc.  It's all part of what makes Magnum Memorabilia our 
all-purpose information clearinghouse.  :-)  For a couple years now I've
been jotting down ideas of more things to watch for in another upcoming
run-through.
As I recall, it was John Hayes who came out with the idea of counting how
many beers Magnum drinks.  We were college undergrads, after all, at one
of the nation's top notoriously "party" schools.  Beer, quantities, and 
talking about quantities of beer all were very significant things to us at 
the time.  (In an only slightly more mature way, they still are.)
Best,
David Romas
Magnum Memorabilia
(MPI's nonprofit foundation for collecting,
production, scholarship, and fan services)
Thank You, Shawn!  As I was creating my little follow-up yesterday I
was trying to remember who it was had told me they incorporate 
Dusseldorfer into their MPI fan gatherings.  :-)  I don't know why I
ever got it in my head that Dusseldorfer is brewed in NY or PA.  I even
have one of the bottles in my beer bottle collection but was too lazy
to go look it up.  :-|
Best,
David Romas
Magnum Memorabilia
(MPI's nonprofit fan organization)
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
"I'd like a beer if I could get one."  - T.S. Magnum "Limbo"
"A pint of stout.  I'd like a pint of stout."  
- T.S. Magnum "Infinity and Jelly Doughnuts"
HIGGINS:  If you're going to drink yourself into a stupor, wouldn't 
          whiskey be more effective?
THOMAS:   [handing over an empty beer bottle]  It's too hard to keep an
          even buzz on with whiskey.
                                                         - "Mac's Back"
I guess, David, you will have to begin production of Old Dusselldorf at
your local BOP.  (Brew On Premises)  Do you think that you will get into
trouble for shipping beer to fans through the mail?  (Just kidding!)
Hey, actually, why am I telling this to you...
Shawn Buchanan Greene