One place I would suggest looking is KEH, an online used gear dealer. A search for MD fisheye on their site currently (as I write this post) yields nothing in the price range you're looking for, but they do seem to have 4 16mm/2.8 lenses in stock, at higher prices.

Another option would be to go and check thrift stores or street/flea markets in your area... This is an area where persistence can really pay off. Maybe you'll get lucky and find something right away, or maybe it will take a while to find the thing you're looking for in this fashion. Still, it's an option.

Another option is to search on the Craigslist site for your area. Yet another is Ebay -- I see a couple of options close to your price range, at the moment (i.e. under $200). Of course, with either of these, you'll want to be careful about who you're buying from, quality, etc. The same goes with the thrift and market options too, I suppose, though perhaps slightly less so, since in those cases you can usually get your hands on the equipment before buying (also true with Craigslist, actually, but probably not Ebay).

Happy hunting! Good luck!