Some folks would use old Computer Power supplies.
I use a cheap end DC power supply as the fans draw next to no power. Even the lowest end power supply (assuming it is the correct voltage as per what is printed on the fan - Either 5V or 12V) which you have at home already would do the trick. Make sure to get the polarity right so the fan spins in the intended direction.
Any old charger (mobile phones etc) will work, just make sure they are at the same or lower voltage and amperage than what is printed on the fan. The polarity, as mentioned, will make a difference.
I have been using a redundant nokia mobile charger fpr the past two years now, still going strong. just hooked it into the fridgemate so when fridge compressor comes on so does the computer fan..
If you find the fans dont run properly or want to just use 5v without a controller just flick the molex (plug) off and just use the wires with the metal end bits in the adjoining holes.