PC fans come in 40, 60, 80, and 120mm sizes commonly. The larger the fan blades, the more air they can blow at a lower rpm, and thus quieter they become. Properly constructed 120s are nearly silent, but the smaller ones can get a little whiny sounding.
PC fans are DC so they will require a power supply. PC fans run of 12 volts so you'll need a power supply capable of 12 volts. Typically the fans are low-current which means most 12V transformers will work. You should double check the current rating on the fans and make sure that the sum of the currents of your fans is less than the current rating of the transformer.
If your fans try and draw more current than the transformer is rated for... blown transformer .
Anyway, you can pick up a transformer cheap from household devices (things like chordless phones are commonly 12V), or you can probably buy one from a local electronics store. Every transformer should be labeled on the back with a voltage and current rating, so no matter what its original purpose, it can be re-used for your grow room.
Then, you just connect the positive wire of the transformer to both the red wires of the fans, and the two black wires of the fans to the negative wire of the transformer, plug it in the wall.
