If you have any old switches laying around, you can use those. I'm talking about old reset switches, turbo buttons, etc.. As long as it can bridge the two pins on the motherboard, it'll work. I did this when I swapped an old AT power supply with an on/off switch for a new ATX ps -- I just ripped the reset switch from an old AT case I had in the basement and voila, power switch.
As for the on/off switch issue, pre-ATX standard computers have been using hard power switches to turn computers on and off for ages. Unless I'm missing something here (which is highly possible), it should be okay.