Need more data.
Is the receiver in cabinetry, is there a shelf above it, or is it out in the open? Could a laptop cooler fit underneath? Are there cooling slots on the bottom?
Noise issues? Is the receiver right in the theater, or is the equipment rack somewhat isolated?
Are yu looking for a simple muffin fan, a squirrel cage blower, something vented, or just a fan to move some air in general?
Muffin fans are available online at Frys and Radio Shack among others.
Could you adapt a PS3 or XBOX 360 cooling adapter?
If you are in a rack, the best solution is to use a squirrel cage fan and aluminum venting to put the air where you want it. Squirrel cages are best found at HVAC supply and electrical supply stores.