Hi CU_AMIGA,
To make the ATX PSU switch on, connect the green PS-ON wire to ground (black wire).
Being an ever helpful person, I have a guide on this subject:
Amiga PSU guideScroll down a bit for a picture of my cable.
Can't find any AT PSU suppliers. I've seen them as 2nd hand items at local computer fairs though, worth a look if there are any in your area.