If you're looking for one for Radeon 7950, 7970 or 280xs you have to get one that specifically supports those models. The reason for this is that the GPU is installed at a 45 degree angle and coolers not specifically designed to fit those cards won't. But coolers designed for any of those cards will fit all reference and most non-reference designs of all of those cards.

That is, unless you're into manually modifying the contact surface. Then you can buy anything. I've managed to cut quite a few GPU coolers to fit cards they're not designed to fit in the past, but the higher end Radeons are a pain.