Look at these they are USB PC fans so you can buy one of those USB block chargers to power it. It has a built in fan speed adjustment which works great. I have mine set at the lowest setting and temps are a perfect 88.4 with 8 - 6qt tubs in the cooler.

Thermaltake AF0007 Mobile 12cm USB External Fan - 120mm, Sleeve Bearing, Manual Fan Speed Adjustment, Retractable USB Power Cable

http://www.tigerdirect.com/applicati...FeHm7AodyUgA-w