I don't think there are more than two alternatives -- one Minolta (16mm), one Sigma (15mm). Provided you mean full frame fisheyes. There is a Russian fisheye too that can be adapted to Maxxum mount, but it's not the safest bet. The Minolta is.
You can buy the Russian Zenitar fisheye from Ebay at about US$100 (seller name ustas http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=707&item=3822919371 ) . However it is a M42 manual focus lens, so you'll need to buy the adaptor ($25 from TCW). Also the aperture pin has to be glued down for the aperture to close.
