I found someone to help obtain R400 cards for our machines.
They will not be cheap.

Original Mac cards (X800Xt and X850XT) are your best bet.
If support for PCIe G5 Macs was ever introduced these would be relatively easy to obtain (at fairly low prices).

Converted FireGL X3 cards are available in limited quantities (but may cost as much as $180).

Converted X800XTs will be cheaper (although the cheapest are All In Wonder cards which will have tv tuners we can't use).

I am also working on a way to convert PC X850XT and X850XT PE cards (PEs are the fastest R400s).

I'll have the first working cards by the end of the month (Feb).