There are some no-name TFX supplies on Amazon, and a few OEM HP and DELL Supplies (but who knows what connectors are on those?)...I saw some in the 400-450 Watts Range, but given how tight the space is in that case, I'm thinking they might be a bit too long to fit.
I kinda think you're headed down the right track with ITX...it's much more popular, and you have a lot more options...still think power and cooling are going to be your biggest hurdles though!
Like I said, you don't pick easy things to build! LOL