Check out the Rosewill cases from Newegg. I've used a couple for low cost builds and the cases are surprising good. You can get a get case with good cooling for $50-70.
Look for the Smart One, Challenger, Armor, and Cruiser. Those are all models I've purchased. I've use the Armor model several times. I haven't done either of those cases with a 6950 so you'll want to confirm the length of your cards. GTX 460, GTX 560, GTX 570 fit fine and so does a 6870.
Pretty much any case will do, there's so many places you can slap fans on, it's mostly a motherboard issue if you can't cool two cards in crossfire due to spacing.