some people (Including me) have sucessfully converted an ATX PC Power supply to run on a powermac G4, with some modification, you might be able to get one to run on a G5, the disadvantage to this, id that ATX Power supplies don't have the 25V lead needed to run the ADC Displays, but with some cable cutting and rewiring, wot may be able to stuff the guts into a G5 Power supply, here is the link to my youtube video of my modded power supply:
http://www.youtube.com/watch?v=LWamNPvIF48
I wouldnt reccomend this as a permenant soultion, but for the time, it was alot cheaper then buying a $200 power supply.