Getting a fan depends on two factors, size and voltage. I don't have that amp, and the schematics on this site don't show a fan (at least that I can see), so I cant tell you either really. But you can find out yourself! Here's how:
Take the fan out (again, make sure things are unplugged), and look at the middle of the fan. It probably has some sort of writing on it as to volts, amps, or something. Write this down. If there's nothing on the fan, try to trace the wires back to where they get to a power source. If you're good with electronics, you should be able to tell if its hooked into the AC or DC circuit. If not, then post a pic here and we'll try to figure it out with you.
Anyways, if/after you know what kind it is, measure it in inches AND millimeters (some fans are sold by mm, don't know why, just one of those things).
Then I would look at digikey.com, or another big electronics parts warehouse for a fan that has the same dimensions and voltage and current (amperes) ratings. Also look at cf/m rating (cubic feet per minute), that tells you how much air the fan will move. Might also have a dB rating for ambient noise, but I'm betting most are pretty quiet.
Cost will vary depending on size and whether its AC or DC.
