Right now I have an old Radio Shack DC power adapter. I cut off the end, ran the wire in thru the Sanyo drain hole, and wired it to the fan. The adapter has selectable voltages on it (12, 9, 7.5, etc.) so I have some control over the fan speed. I've heard of others doing the same with mobile phone chargers - pretty much any AC -> DC adapter will work for a DC fan as long as it has decent voltage (PC fans are usually 12 volts).
Ideally (and maybe eventually), I'd wire the fan into the fridge wiring so it would be powered by the fridge and only run when the compressor is running. Or I might wire it into the fridge power so it stays on all the time - I'm not sure if that's necessary though. If I go with either of those options, I'll get an AC powered fan.