On the GPU is a heat sink. It is not designed to have a fan attached to it.
If you want to have a GPU with a fan attached you will have to replace the heatsink as well to one that accommodates a fan.
But, unless you are overclocking your Xbox, what point is there to this? The GPU will run at standard operating temps, the CPU would probably overheat first, and all you are doing is making more noise in the Xbox ...
For fans and general modding supplies, I used to use
http://www.so-trickcomputers.com/ (dont use them anymore only because I am not modding my PC anymore) ... they had a good range or modding supplies at reasonable prices. Suggest panaflo fans if you can get them - low noise, high CFM. Not the lowest noise, or the highest CFM, but a very good compromise.