You can't buy GPUs for Apple (or almost any other) laptops. They are surface mounted to the logic board. The only fix is a replacement logic board which will have the GPU, CPU etc surface mounted on it.
Had the same problem last year, luckily I was in the last year, or month, of Applecare and showed them this picture:
Apple replaced the entire logic board free of charge. This is also a common problem (http://www.crosspond.com/apple/hall_of_shame/1). Maybe if you complain enough, show them the site like I did, it'll work. Hope it helps.
fixing it isn't easy. there are a few ways to try it - but the only real way to fix this is to replace the gpu. maybe you should look at the ebay number 250665317821 (put that number into the search box) - i think it's worth a try