Don't steal power. It WILL get you busted or dead. If you are unsure of how to wire a couple of computer fans then you should never NEVER mess with the incoming power.
Mosst computer fans have a bacl wire and a red wire. Black is ground and red is 12 VDC . You can use a computer per supply. Once supple could run 10 fans no problem. Just make sure that you run them in parrell not series.