You can't install an older version of MacOS on any Mac that originally
shipped with a newer version pre-installed, so that means you're stuck
with buying an older, second-hand Mac. You would have to avoid anything
that originally shipped with Catalina or Big Sur, but according to
Apple's website:
These Mac models are compatible with macOS Mojave:
- MacBook (Early 2015 or newer)
- MacBook Air (Mid 2012 or newer)
- MacBook Pro (Mid 2012 or newer)
- Mac mini (Late 2012 or newer)
- iMac (Late 2012 or newer)
- iMac Pro (2017)
- Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with
recommended Metal-capable graphics cards)
<
https://support.apple.com/kb/SP777>
There are some exceptions listed on that page where you will need a
newer computer for certain Mac OS features.
A good app to check which OS shipped with a particular Mac is the free
MacTracker. <
https://mactracker.ca>
I can't think of any, but it's potentially possible that there could be
an issue with some models if they have already been updated to a newer
version of Mac OS which required a Firmware update, since it may not be
possible to downgrade the Firmware again.
The other option is to just use a newer computer / version of Mac OS
and run Mojave and the 32bit apps as a virtual computer under Parallels
Desktop, VMWare Fusion, or similar. That of course does need more drive
space and RAM since you're using two versions of Mac OS at the same
time.