iMates appear regularly on eBay from what I've seen, and aren't *too* expensive.

USB is 5V, and generally only delivers 100 mA unless a device asks for more power. I don't think this will be enough power to start turning even a small 12V fan, but it could be worth a shot.

Personally I would much rather go for a larger 120 or 140 mm fan undervolted to 5V off a beefier external power supply. This is likely to be both quieter and provide much more air movement.