Most PC fans run at 12 volts, (although as Hammy says, you can sometimes find 120v ones). You need a low voltage source, which you won't find in your fridge.
Easiest is to take an old "wall wart", like a cell phone charger, laptop power brick, or something else that puts out around 5-12 volts, (look at the sticker to see what the "output" is). Cut the plug off of whatever it is and connect the wires to the wires from your fan. Hopefully, there's only two wires coming out of your power supply. There's no "wrong" way to connect them....connect them one way and the fan will run in a certain direction, reverse the connections and the fan will run the other way. Experiment to get the fan to spin the way you want it to.
Edit: Derp. Just re-read Hammy's response and now see he also references low volt fans. Oh well.