I use one of the old chrome supplies that I pulled out of a 386. It's an AT supply and has a huge red switch on the side of the case. It sounds like you have an ATX. first go here and look at the pinout:

http://xtronics.com/reference/atx_pinout.htm

What you want to do is, at the connector, cut 13 (black, GND) and 14 (Green, PS_ON). Strip the ends, twist and solder them together, and the wrap the bare ends in some electrical tape. Now the supply will turn on automatically each time you plug the supply in. Then you can use any one of the hard drive/CD plugs (Red, Black, Black, Yellow) to supply you with your 5, 12, and Ground. Good Luck.