I'd suggest consulting Sound Doctorin'
( http://sounddoctorin.com/synthtec/parts/key.htm) . It is the largest
replacement key list I have found.
If you have your broken key, you can compare it to the pictures available and
determine if an alternate key (Matrix-6 OB-8 Pratt & Read ) would fit.

Please share your findings when you find one. My keyboard is fine, fortunately,
but you never know.


(I'll save this for a separate thread: I was toying with the idea of painting
keys to have a reverse color keyboard...)