Okay. Don't take my word for it since I have no interest in water cooling, but I would take a look at DangerDen. They're a little pricey, but they seem to come recommended a lot.
You may also want to make sure you really want a GPU water block. A decent one (that covers RAM and power management) for a high end graphics card runs at least $100+, and waterblocks are usually not compatible with anything else down the road. And chances are if you want to pawn it back, you'll have to put the original heatsink assembly on it.