You'll either need to modify a standard ATX power supply for the custom pin configuration of the iMac logic board, or acquire a power supply that is specific to that motherboard.
Here's a couple places to start:
http://www.cryogenius.com/hardware/imac/ http://www.macmod.com/forum/showthread.php?t=425
A simple Google search will turn up what you need to modify an ATX power supply for the iMac.