I've been looking around for a small fan for my extruder, but most of the cost about $10 + shipping which I felt was a bit too high.
But dealextreme came to the rescue with a replacement fan for the Nintendo Wii. It only cost $3.60 including shipping, and I just received mine. It is not the most powerful thing, and I have yet to mount it, but I thought this might be of interest to others.
Dimensions are 35x35x15mm and it runs off 5v. The black lead is negative/ground, and the gray is positive. It comes with a small connector which you remove by lifting (hacking off) the tiny tabs. There are mounting brackets on the side, but they can easily be cut off if needed.