This place has a nice variety of units. Most have thermisters that turn the fan on automatically when it gets warm so you just leave them plugged in all the time. I have a pair on my HT amp that do a good job and are pretty quiet, but I can slightly hear them when nothing's playing. Or better yet find some nice wood grills and "open up" the sides of the cabinets so you don't need the fans.
http://www.activethermal.com
http://www.activethermal.com