you may try transtar, I think they are the biggest supplier of trans parts for rebuilders. https://www.transtar1.com/

A couple things 904 transmissions have not been built for a long time, so they really are antiques. The A500 is an overdrive that was built more recently, and the main section is the same as a 904, you may ask for snap rings for a A500 and get better luck finding them. I still build a few transmissions and a lot of the younger sales people who don't know about older stuff will tell you no, when if you look it up yourself the part is available, they just don't know the history and interchange of the units. Kind of makes me mad, but many times I just look it up myself now and ask for part numbers and not even try and have them look it up. Sometimes they pester me for a model year and application, and when I say something like, a 1964 dodge, with a 540 CID, etc etc. Their eyes kind of gloss over.

Another thing if when you say rear clutch, you are referring to the drum that has the input shaft attached to it, you can fudge on the plate clearance a little, that clutch is applied in all the forward gears, the only time it has to freewheel is in reverse. As long as the clutches rotate when when the snap ring is in place normally works fine.