From your post I assume that the card you mean is the combo Airport/BT card, correct?
Only three PowerMac G5s had that combo card and those are the 2.0, 2.3 DualCores and the 2.5 Quad. The combo card is is the same in all three. You said your Mac is a Late 2005 G5, so it's either one of these models. If it's aircooled then it's a DualCore.
In any case, the combo card works in all three and these are the only three models that have it. So, shouldn't be hard to find. Be careful though, I believe the early MacPros also had this and I don't believe those are compatible.
Did the person you bought the G5 from leave the ramp card in the Mac, or take the entire thing?
The ramp card is the part that the combo module attaches to and that actually plugs into the slot on the logicboard. Those are more expensive and probably that is why the seller took it.