Well, I don't mean to burst your bubble but rare does not always equal desirable. I don't have an production numbers to back it up, but I can tell you that the 2bbl 350 was the low end V8 in the LeMans and the 3 speed manual was the base transmission. Is it a rare combo? Possibly. Is it desirable? Doubtful.
The reason you probably don't see a lot of 3 speeds out there is the same reason you don't see too many 2bbl or 350 cars still running around. The 350s have been replaced with a 400 or 455, the 2bbl went along with it in favor of a 4bbl and the 3 speed manual has either been replaced by a 4 speed or a TH400. Good luck in your search.