If you're looking to get one cheap, look for kit auctions where the seller doesn't understand he has a 2.8 rather than a 3.5. It's pretty easy to see the larger front element in even a crummy picture. But of course this sort naivete means you won't get reliable condition info, and fake naivete as a means of unloading junk is far from unheard-of.
And, FWIW, if anybody has a 100/2.8 with bad glass or shutter, let me know, I've got one where some idiot epoxied (rather than the standard and reversible nail polish) the adjuster screw, and the "professional" repair failed in such a way as to destroy the brass rim that drives the rangefinder.