I just printed a holder for a 40mm fan, ordered the fan off Amazon and should be here on Saturday ($6 shipped).
These are just standard CPU/PC fans, and have a 3 pin connector with red,yellow, and black wires. Red is your positive voltage (could be anything up to about 12V). Black is negative/GND. The yellow wire is a tachometer/RPM sensor.
This color scheme can be confusing, because usually yellow wires in a computer are +12V.
They are variable fans, meaning the more voltage you supply, the faster they turn. Motherboards control them by turning them up and down as needed to match a set RPM.
If you hook the red and black up to constant voltage, and ignore the yellow, it will just run continuously.
More interesting would be to hook it up to the fan sensor on the extruder, and be able to control the speed with G-code. I don't know for sure, but I think the extruder fan runs off 5 volts, so you won't get the full power of the fan.
Someone else who has done that mod might be able to chime in.
Da Vinci 1.0a. Repetier 0.92. E3D V6. Titan Extruder. Sllic3r. Octoprint on Raspberry Pi B+.