Looking for G4 replacement parts

I just recently got a G4 500mhz tower from a friend who was selling it
as "unknown" working status. After some testing and swapping parts with
another G4 system, we realized that the mainboard in the system is in
fact bad. Does anyone have one of these (or even the whole system if the
price is right) for sale? I'm just looking for the replacement board.
Apple wants ~$490 for one, so I figured I should try here first. Also,
if anyone has any recommendations on a good place to find one (besides
eBay of course), I'd appreciate it.

-Mark Astrauskas