Can you buy a quiet fan for amp?

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.

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

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