Hi ihorkor,
I found this one on eBay, just now, searching for NVIDIA GeForce GT130 for the 2009 iMac 24" (the subject of this particular page on ifixit:
http://cgi.ebay.com.au/24-iMac-Early-2009-NVIDIA-GeForce-GT130-Video-Card-/170579003844?pt=PCC_Video_TV_Cards&hash=item27b74d09c4#ht_1330wt_706
I think the following card better suits your 2008 24" though:
http://cgi.ebay.com.au/iMac-24-inch-Early-2008-ATI-Radeon-HD-2600-Pro-USA-/370480748699?pt=PCC_Video_TV_Cards&hash=item56425f989b
Cheers,
Charlie @t ewhizz dot net