I use Silencer fans. You can't hear them when they are on, and they are very cheap. They run on 12 volts though so you need a power brick to run them. PC Power & Cooling
I was looking for one too but I prefer to get a 110-120V fan just so i don't have to buy another power supply. Are these usually much louder than the 12V fans?
Also, are there any adapters available to convert 120V AC to 12V DC that will work well with these?
A power brick from an electronics store (like Radio Shack) that is rating at 12 volts and >250 millamps will do fine. Just cut the end off and wire it up to the fan.
I use a small desktop fan from Brookstone with my A/V receiver. The fan was $30 at Brookstone a couple of years ago, and I bought an AC adapter at Radio Shack for around $10. I'm not sure if Brookstone still sells the fan. If they do, just make sure you get the correct size AC adapter pin at Radio Shack to fit the input on the fan. When I bought the AC adapter, the pin was free. I just had to choose the correct size. Anyway, this fan allows you to adjust the speed, and I use a lower speed that renders the fan silent.