I have a very nice Mamiya RB67 37mm fisheye lens for sale. The glass is clean and bright, the shutter and fstops are in fine working order. $525+Shipping
I have no affiliation with the OP, but I too have this lens and I basically shoot everything with it. It's sharp, clear, and will catch your toes if you're not paying attention. It Is by far the best lens I've got for my RB system and I love it. Anyway, just thought I'd chip in my .02 on this great lens.
I have the RZ version also. Great lens. Something about the projection or something makes it seem less fishy than other 35mm format fisheyes I have used.