The GEForce FX 5200 is relatively available, as this was the "stock" card in many G5 systems.
The Mac edition of this card has a blue circuit board and is passively cooled(i.e. no fan on the GPU). It will have two ports-one a standard DVI port, and the other an ADC(Apple Display Connector) port-this looks sort of like a DVI port(and actually mostly is) but has a sort of oval shape as well as "hooks" rather than screw standoffs to secure the cable.
I just checked on Ebay, and there are a couple of Mac-edition FX5200s available for $10-20
Overall, this is not a great card-especially for OS X 10.5-but is workable.
Depending on how much you want to sink into it, there are better cards available, although the Mac edition of many of them will set you back a fair bit.
Here's a full list
http://www.jcsenterprises.com/Japamacs_Page/Blog/4B4B7BA2-7ABB-47F1-87AC-B03D37942BEE.html
I have a GEForce 6800 Ultra in one of my G5s, and have been incredibly happy with it. I paid about $50 for it(Mac edition, not flashed). As this was a build to order option, it's probably the most plentiful "high end" OEM G5 card. The Mac edition X800 and X850 generally bring well over $100. By the way, Mac edition X800 and 850s have one ADC and one DVI port(the 6800 Ultra has dual DVI).