one can use the AC to DC adapters that you get from broken appliances (or you can get a wallwart) and plug it into the switched outlet of your receiver
the 70/80mm fans will run even with lower than 12v current (even if you wire two of them together)
in fact, i purposely use lower than 12v to make them spin slower.
but the 120mm ones will spin too slowly if you don't give it 12v (i tried 11.3v)
i use a lighted fan because it helps me remember to turn off the AVR
and i have a very small clearance between the top of the receiver and the top shelf