You're right HiTechObsessed. Actually it looks like although mini-ITX is a standard for motherboards/cases, there isn't a standard size power supply i.e. there is no standard fit. From browsing around, they are go by:
mini-ITX
Flex-ATX
1U ITX server
and a few others
Unfortunately almost everything in this size is 300W or less. So far the closest match I've found is the Athena AP-MFATX40.
This isn't as long, not sure if you need that, but it's the same width, and a hair shorter in height. I think you're looking for a mini ITX power supply