I'd recommend the
Fujipoly ones from FrozenCPU.com. They're really nice, and work great. Everyone who's used them has loved them.
There's 3 things to look for when buying them, listed here in order of importance:
1) Thickness:
Most common are 0.5mm and 1.0mm. I used 1.0mm on the RAM chips, and 0.5mm on the VRMs, but it will depend on your specific GPU/HSF, and whatever it used originally. You can get some thicker stuff like 1.5mm, but I don't think that's used much in GPUs (idk tho, maybe for RAM chips). The thicker the pads, the harder it is for the heat to move through them into the heatsink, so you want to buy as thin as you can get away with, while still maintaining good solid contact (aka the pads are being squished a moderate amount).
2) Thermal Conductivity:
Those Fujipoly ones come in 3 grades: 6.0W/mK, 11.0W/mK, and 17W/mK. From a technical standpoint, the higher the number, the more heat it can move through the pad, and into the heatsink. Those 17W/mK are about the best you can buy, esp as a consumer, but it's not exactly cost effective. I use the 11W/mK ones, and have had no issues. If you're underclocking your RAM while BTC mining, you could even use the 6W/mK ones, but it really depends on your application.
3) Pad size:
Most of the pads come in sheets, and you can cut them into the sizes and shapes you need. Make sure to have an extremely sharp razor, and don't drag it over the pad, as it can grab and rip. I think I picked up the 150x100 sheets, but you can get bigger or smaller sizes depending on the amount of hardware you plan on using them on, and your budget.
Hope this helps.