advice naman pano setup ng power source pag molex not from usb port.
Any DC power source from 7v to 12v is enough to make the fans spin. Higher voltage means faster fan speed.
So an AC-DC adapter with variable voltage, say 6-12v DC, and 500mA current is probably enough, though check your fans if they need more current.
Alternatively, you can use power adapters that come with USB-IDE or USB-SATA adapter kits.