If you just need a power supply to run the drives, you can get an AT power supply. They have a hard-wired switch, so you don't need to fool around with rigging up a switch to do it. You would have to find a place to mount it unless it is the type with the switch on the back of the supply itself.
If you want to go ATX, it might be easier to find a location to mount the switch. All you have to do to turn it on is momentarily short 'PS-ON' (pin 14). You can get a single-throw momentary switch anywhere - RadioShack, Fry's, etc... Or, it may be still connected to the case. Then all you'd have to do is connect one lead from the switch to pin 14 on the ATX connector, and the other one (ground) to any black or ground pin on the ATX connector. There are AT and ATX power supply extensions that you could use to make a neat job of it. Just cut off the female end of it and make your connections there.
Either type power supply should screw right in to the case, and should be available at any commodity PC parts supplier. CompUSA, Fry's, etc..