Maybe I've just had a bunch of bad luck when buying "used" parts stripped off disassembled firearms, but quite often they can be used as a pattern for making a new part:
Round firing pins are the most easiest to fabricate, and if you like to live on the edge, some round stock can be spun and filed to shape using a drill press. I do not recommend that process, but it has been done.
A good, skilled machinist, if you have one nearby, just might be willing to get involved with one of your projects should an obsolete part be similar to trying to find a Playboy Bunny in Iraq.