I managed to find another power supply. It doesn't work right either, but it does work a lot better. It's much easier to get it to turn on (just leave it for a minute after plugging it in and it works every time), but it rarely turns off properly. The old PSU is a Sony, and the new one is an Astec.

For the record, an ATX PSU can be made to fit, but it would have to be a mini-ATX PSU. It also would have to be stripped from it's housing and jammed into the old PSU's housing. The only real problem with getting it to work would be the soft-power circuit, which is probably very different from what the Mac uses.