Kevyo is right. You want a low-speed one for two reasons: prevent problems with tubes and low noise. Rogue uses one that is amazingly quiet and you may be able to find out from them what brand it is. And don't blow it direct on the tubes.
Looking for Silent Fan Cooling f my ARC Tube AMP
I have my ARC tube amp in a corner. I like to stick a fan behind it . The one from radio shack worked great but made a little noise. Any suggestions . Thanks to all !