I think there s a place in AU that rebuilds them. Other than that I don't there's much for options. I'm thinking about converting to drum setup with an aftermarket disc conversion. itd end up being the same no ey in the end but I'd gain parts availability.
Rare parts has them or can rebuild yours in California . Expensive but no other option . NOS ones are now near non existent . Rare Parts Online Catalog, SPRINGS & COMPONENTS
Probably the "rebuild" is just injecting the loose and worn out assembly with a liquid teflon/plastic composite. Will last for a few years then fall apart again.
Check w/ member hemi71x on the A-body forum. He has or had some refurb'ed early C-body disk setups (I recall on craigslist). If anybody knows where to get the ball joints, he would.
The easiest way to deal with 4 piston Budds is to put them on a shelf and use 69-73 style discs. All you need are the spindles and "regular" lower ball joints.
You can probably do the conversion for less than the price of those unobtanium ball joints.