Aside from calling e-machines, www.newegg.com
tends to have some of those off-the-wall parts. Was the old power supply proprietary? Because I've seen some old compaqs that had power supplies that looked like a standard ATX, but had the lines all switched around so you'd have to buy a replacement from them. I'd check into the pin arrangement on the old power supply, and then if they are standard, any micro or mini ATX might fit.
I don't know what it's like now, but when I was working retail (three or four years ago), most of what I saw for COTS computers was proprietary. Standard connector, but different pins. Kind've hard to know what you're doing if you can't jumpstart the power supply and read each line with a multimeter, though.
Pink's right. Find out what the pinouts are. Then jump the power supply; if it doesn't come on, your power supply is shot. If it comes on, check your 12V and 5V lines to see that they are within spec.