I haven't done it myself yet, but I've been reading up on just that, because I'm going to wire a pc fan soon to my little grow cabinet that I'm in the process of building.
I found out that most pc fans come with 3 or 4 wires. You're only interested in the red and black wire. You take a spare DC adaptor lying around the house, like from a cell phone charger that's around 12 volts, and you just connect the 2 wires (red and black) from the PC fan to that. You would splice the wire and connect them. Maybe tape over the connection with electrical tape. You can also get one from a place like radio shack if you don't already have the DC adaptor wall wart. Most people have a few spares laying around some place I think.
At newegg, they sell 10 cheap pc fans for 10 bucks. Check out the CFL rating. The higher the CFL rating, the more air the fan pushes. Also, check out the DB rating, if you're interested in the noise issue. The higher the DB, the more noise the fan will make. So basically, you want a fan with a decent CFL rating and low DB rating, if good airflow and noise concerns you.