Sourcing a replacement graphics card for an iMac
There were three gfx card options that went in this model:


NVIDIA GT120 (256Mb): part #661-4991
NVIDIA GT130 (512Mb): part #661-4990
ATI Radeon HD 4850 (512Mb): part #661-5135


Searching for these cards by this "Apple Part Number" yields many online sellers. Ebay, macpalace.com, powerbookmedic.com, macpartsonline.com...

(How to get the apple part number for any given computer is a mystery to me, so if someone wants to elucidate, I will probably accept the answer.)