Here's one of the few fan controller circuits I like (Mainly because it doesn't require the purchase of a ranged thermistor)
For a little clarity on the components: Q1 doesn't HAVE to be a BD140, just about any power transistor that'll sink what your fan draws +10% is a good choice. I'm running somewhat small fans, so my transistor is a 6.5W design. Of note on this transistor, please heatsink it. It will get warm with use. For those who don't know, the odd symbol for D2 designates a Zener Diode, and since the writer of this original schem was australian, 220R for R5 and 10R for R6 designate 220 ohms and 10 ohms respectively. Kind of reminds me of the confusion with the people who thought 2.3k and 2k3 were different values
but I digress...
The 'Diode Sensors' are basic 1N4004 diodes, available... Well, anywhere really.
I got the circuit idea originally from Elliot Sound Products in Australia. This is their original graphic, so I hope they don't mind too horribly that I'm posting it here. It's a decent, simple design. Albeit, it has no voltage control to the fan, but considering the location of the fan within the circuit, it would be of no consequence to wire in a voltage controller.
All components, except
possibly the uA741 should be available from your local Rat Shack.