By as-is camera lots on Ebay. I picked up four Fed2's, Fed 1, and a Zorki C for $30 inclusive of shipping. Repaired a number of cameras with the parts. The RF trim rings of the Fed 2 fit a Canon bottom-loader perfectly.
Some of these set screws are triple-zero size. Hard to find. Some seem custom made for the lenses. I've accumulated parts for J-3's and J-8's, out of parts lenses. Seems they are all one-of-a king screws on these things. There are exceptions, I used screws from an Olympus RD to fix a Canon 50/1.5.