Buying new LP for cheap, forget it, unless its ancient cast iron Sovjet stuff.
Ebay is one option.

Some roboteers have/had a small supply of lp stuff. But nine is really gone now? I maybe have a valve or 2 left, and a tiny piston somewere. But thats about it.

A 50*50 ram is way to small. A 50*100 is usefull.

Cutting a round rack involves a precision mill, the right cutter and a lathe to finish it off. Meaning an experienced machinist and machine shop. At this time, machine shops dont come cheap. They all have enough work.

I solved the rack for the FP heavy axe I did build by buying a mod 3 round rack and build the ram around that part.

If I would do a rebuild of a 50*100 ram I would buy a mod 2 rack and replace the original rod with that. ( mod 2 because the rod is 20mm, and that wont be to far off for a 50mm bore lp ram)