There should be many places you can find one.Maybe even this sight.2 and 4 track machines seem to be more plentifull than 8 or 16 tracks.I would just keep searching by brand name, format, etc....
A good 8 or 16 that's in good shape isn't going to be cheap,but you never know.I paid about $320 for a 2 track Akai GX 266 but that included manuals,shipping and proper set-up.Try to buy from someone who works on them.I think it's better to buy from a person who really knows about the unit and pay more than to buy a mystery machine and inherit a lot of problems.
With the advent of Ebay, there really aren't any "deals" out there anymore. Many so-called "power sellers" are pawn shops, which aren't always good places to buy from.
It's true that buying from a good repair facility may be your best choice, but they sell usually at blue-book value, often times higher than the current market price.