Good luck DMan, I'm going thru same thing; all Powermaster parts are very rare & from what I've seen most of it is junk taken off cars that were either stripped long ago or left overs from a brake conversion project.
Dealerships or GM distributors, anywhere, don't have any new parts. If you can find something, It'll be old & it ain't cheap. I could not find rebuild kit, so bought a used master cylinder from GBodyParts (I think owned by BWeavy) who is a site vendor & posts sometimes.
May be time to consider a conversion, may be even cheaper in short-run & will be able get parts in long-run. Good luck...Ray