Vintage Synth Repair

I've got a Prophet 5 keyboard on my bench.  It's quite straightforward
(and crude . . .).

The bump stops for the keys have hardened, crumbled or are otherwise
disintegrating -- to the point that it is difficult to determine what
material was originally used.  What material would be best for
replacing those bump stops, and where can it be obtained?

Thanks in advance.

Thomas