> I called Sonnet and asked about their Crescendo 7200 upgrade

The Crescendo 7200 is completely irrelevant in a MorphOS context. It's an upgrade card for pre-G3 PowerMacs, i.e. with PPC601 "G2" processor, providing 400 MHz G3, 500 MHz G3 or 400 MHz G4.

http://www.sonnettech.com/product/crescendo_7200.html

> it appears at least that one, the PCI version

All Crescendo 7200 models are to be plugged into the PCI slot, there's no exception.

> is not available anymore, even though it says it is on the
> website, when you go to actually "checkout" your cart, it
> gives an error message saying it's not available.

Yes, that's the 500 MHz G3 version.