What hardware is going in it?
One of these would almost fit in that space (3.2"x1.7"x6"), but the biggest one is only 300W. I think I've seen a 330W of that size before, but don't remember where. A
1U PSU would fit (3.875"x1.5"x7.5"..the one I checked anyway, check the size on one before you get it, sometimes they're really long), and Newegg has them up to
700W (though that one is 9.84" long..and only has 16A on the 12V line). Another option would be to get
one of these small redundant PSUs, pull out one of the modules, and rewire it. The full units are about 8x6x3.5, so you would probably end up with something about 6-7x5.5-6x1.5-1.7, and Newegg has them up to
658W. Alternately, you could do something like what Kayin suggested. I have a 203W XBox 360 PSU you could have for $10+shipping if you want (16.5A 12V rail and 1A 5V rail).