Shit bro, you can get one right now for like 5 bucks at Tiger Direct. It's not to loud and it will adjust to the temperature change in your box.
Masscool 90mm Ball Bearing Case Cooling Fan 09025B1M34 at TigerDirect.com
I'm not sure how to actually wire it up though. I've never looked into it. I'm sure it's not that hard..