This is where I purchased my fan.

http://www.coolerguys.com/

I read that some fans give off heat so they raise the temps in the cooler. You want to look at CFM's also. 12V fans need a converter like a cell phone charger and 120v plug right in to an outlet. If you go bigger you can use a dimmer to slow the fan down. If the fan is too small you can't speed it up.

My problem right now is placement of the fan to provide even temps throughout.