The following site seems to think that you can use junker Sun 4 keyboards
for parts:
Keytronics keyboards turn up on all sorts of machines (I've come across
them on PERQ 2's, Apricots, Tekky workstations, etc). And from what I've
seen the mylar/foam disk assemblies are all the same size
However, these parts decay naturally, whether the keyboard has been used
or not. I'd be a bit careful about spending money on an old keyboard to
get them. They may well be useless, or fail again in a short time.
-tony