TL flag, sorry, I do not know a source, but I think some gunsmiths and home machinists are capable of fabricating screws. Not a skill set with which I am at all familiar, but..
Also, are you certain that a proper sized screw is not currently being manufactured?
TLflag you might check your local Ace hardware for screws. Most of them have a pretty good selection of gun screws. You may have to shorten some but I have used them many times.