On eBay, but mirroring your statement they are an extremely rare find. I got lucky when I saw one, and subsequently blew all my savings on it.
I finally nabbed one! The chassis was $80 on eBay, but I did it anyway. It just arrived a few days ago and I was so excited to paint it!
Dude, love the glowing psychic fist!
All your army looks freaking amazing! #jealous