@X-ray:
It depends where you get it from. Elbox sell a nice, but kind of expensive ATX to AT converter, that is used in Mirage 4000 Pro towers. This one is made by Elbox, and it even comes with some sort of a controller chip on it, as well. You can buy it from their online store. ATX on/off switch is required, too. It connects to the power pins on the ATX to AT converter.
I'm sure that you can find similar and cheaper adapters by Googling around, or perhaps checking out eBay.