The two GPU boards your system supports are:
* ATI Radeon HD 4670 (256 MB) - Apple P/N 661-5539
* ATI Radeon HD 5670 (512 MB) - Apple P/N 661-5546
I often find looking up the Apple part number will more likely get you the correct part. In any case the firmware chip will have an Apple logo label on it. Thats the most important thing as some people will try to see you the Dell board which won’t work.
Here’s a few ones I found but they go quickly! You may need to call them.
ATI Radeon HD 4670:
* [https://www.powerbookmedic.com/Intel-iMac-215-ATI-Radeon-HD-4670-256MB-Video-Card-Mid-2010-p-118071.html|PowerBookMedic USA]
* [https://www.thebookyard.com/product.php?products_id=7348|TheBookYard UK]
ATI Radeon HD 5670:
* [https://www.powerbookmedic.com/Intel-iMac-215-ATI-Radeon-HD-5670-512MB-Video-Card-p-25219.html|PowerBookMedic USA]
* [https://www.thebookyard.com/product.php?products_id=7350|TheBookYard UK]