I'm trying to improve ventilation in a home entertainment center and am looking for a general purpose fan. The idea is to cut a hole in the top of the intertainment center and mount the fan. Ideally, it would be small (3 to 3 1/2"), quiet and would plug into a standard outlet. It would be great if it were thermostatically controlled too.

I've searched the internet, been to Lowes & Home Depot, but no luck.

Any suggestions would be much appreciated?

What you want is a muffin fan, commonly found in computers and related equipment. Many run on 110V AC, while some run off the DC power supply of the equipment. You should be able to go to where they sell computer equipment and purchase a new one. Or you might want to find a junked machine and get the fan from it. There are plenty of old computers around that are worth nothing and end up in the trash.