I got mine at
www. servercase.com a LONG time ago. I'm not sure if they sell it there, but I remember it was slightly cheaper there than Directron.
As far as switching the psu, I'm sure it's possible, but you're talking about doing a lot of cutting and then filling the spot where the psu was supposed to go. Plus you'd have to install some sort of reinforcing brackets to hold the power supply up.
There's a little bit of space at the top between the motherboard and componenet chamber. There's room for a mobo power cable for sure.
-Kev
GOOD LUCK! Give me pics when you're done!