Ultra small form factor motherboards?

This is largely for one of my stranger modding ideas, but is there anywhere one can buy the ultra small motherboards such as those used in the MacMini, the one VoodooPC box, and the new Shuttle computer that looks the the original NES? None of those places lets you buy them as barebones, and it seems a little less than cost effective to buy the full computer with the intent of using just the guts.

Even the Flex ATX boards are a little big for what I'm thinking, but these ~6"x6" boards the various companies are using would be great.

EDIT: Damn it, I just realized I used the wrong MoBo section by mistake. I'm so sorry, guys. Please ignore this.