You might check with Alpha Supply at www.alpha-supply.com
they are a jewelry supply house and they have hard carving wax for casting.
that might be what you need. It is quite hard but carves with regular
tools.

Brian, IMO that's the best advice I've seen. Even more so since Alpha
is such a short drive away

There is also a hard maching wax available. It is used by CNC operators to test
a program. I think Freeman Supply has it.

Dan