Graphics Card Replacement for iMac 2009 27"

I have recently returned from a repair store, where they are saying they don't have the replacement part for my graphics card for my I mac and they cant do the replacement because it is an old model.

I have found another tech company that will deal with the repairs but the works are very expensive. I am trying to find the graphics card myself and getting them to do the labour works at a cheaper price.

I am looking for a replacement for the following graphics card i already have:

ATI Radeon HD 4670 graphics processor with 256MB of GDDR3 memory

if any one could point me in the best direction to make this as cheap as possible that would be a great help.

Regards