Can you buy a quiet fan for amp?

I've seen a product called "Cool-It" at a local hifi dealer. It is very quiet (DC motor) and can be plugged into a switched outlet so it turns on when you fire up your system. They make a couple of different models for different placement options (in-rack, on shelf, etc.) and may provide enough airflow to cool your amp. A Google search will lead you their website or where to check them out locally. Good luck!

Fan noise is caused by the blades, so the nature of the motor turning them (AC or DC) is not a factor. It may be that a particular DC fan turns at slower rpm, and that would be quieter. Design of the blades, and whether the fan is small and runs fast, or larger and runs slow is what you care about. No practical fan is completely quiet.

I have a qsc amp, but it lives in the cellar (out of the listening room). That is the only perfect solution.

You might want to hide the fact (from your audiophile buddies) that you use a pro-sound amp. If so, and if the cellar is not an option, consider putting the amp in a soundproof box of some kind, with some provision for ventilation through internal sound-baffled ducts up and down (like a chimney). The amp has overtemperature protection, so you won't do damage.

If the amp has external heat sinks then a ceiling fan would work well.

Last time I looked Radio Shack had quiet AC fans, about 4 inches square with twin leads. Just add power cord!

If you're using an AC fan, try a DC fan. In general,
DC motors run quieter. Another advantage of of a DC
fan is that you can run it at lower voltage, which means
lower rpm and lower noise. Try a 12v fan (common)
running at 9v - just buy some generic wall wart power.
I picked up several DC fans at a used electronics parts store.

I've also seen websites advertising ultra-quiet fans
for computers, but I've not tried these.
For example, http://www.endpcnoise.com/
or just try a google search for quiet computer fan.