If you ask politely here, some people will provide it for free by email.
Far fewer than before the September That Never Ended, but there may be
a few lurking about. I would estimate a dollar or two for most lock
shops. Showing up with the lock is probably a good idea if possible, at
least that way there isn't any question that it's yours.
The older ones are very easily manipulated, another possibility. Newer
ones also can be manipulated but not quite as easily.
Unless you have a use for several of them them and you're buying a large
lot, or they have collector value, buying used common Master combination
padlocks on EBay seems somewhat counterproductive. A new one is, as you
say, about $5. Postage on a single padlock would likely be about $2 at
a minimum.