Items:
One PC fan.
Any size DC batter (like 9v, etc)

I need the battery, to operate the fan. I know that pc fans are AC, but im working on something and a pc fan would be perfect for what I need it for, but I need it to right now be able to operate it with a battery.

I've been told it cant be done, but maybe I just dont know anything about electricty.

I believe pc fans are 5v dc either that or 12 v dc, should say right on the fan.

You should be able to pick up a dc fan for next to nothing anyway, even if a regular pc fan isn't (which I'm fairly sure it is)

check out surplusshed.com they've got a few.. looks like they're all 12volt, but I would imagine you could get a 12v fan to run off a 9v battery, and if not, wire two 9 volts in series and it'll run.. otherwise, I'm sure you can find a 9v dc fan somewhere.