I would use the hard drive power connecter (technically called a "molex" connector). There should be 4 wires on it. One red, one yellow, and two blacks. The yellow is 12v+, the red is 5v+, and the two blacks are both grounds. Simply splice into that line, using the red as the + and one of the blacks (doesn't matter, they are both grounds) as the -.
By your post, I'm guessing you don't have a USB port, but if you do, the USB port provides 5 volts of power you might be able to tap into.