Your over thinking it. Goto Walmart and buy a 12v AC adapter from the electronics department. There about $12 a piece. It actually has a switch to adjust the voltage from 12v, 6v, 3v, etc..
Once you have that your set. Just cut off the pigtail connector from the pc fan wire. On the fan you should have either 2 wires or 3 wires. You want the Black / Red wires. If you have a third wire which is yellow just tape it off (thats the fan speed wire for the motherboard on the PC).
Just splice them all up and twist together. Add some electrical tape to seal it up and your set. If it does not work then you might have to swap the wires. Sometimes on the AC adaptor end its hard to see whats positive or negative.
Hope this helps.
edit: Radio Shack sells them for about $24 so go to Walmart if you have one in your area.