Wow, that is a neat-o thread richard2717 gave the link to. If I were going to do this I would make my own like their forum member
Lynden shows in the 5th post down in the thread,
Hangar Style Bifold Door #4.
The video shows a metal one and a wooden framed one. I think you could take a standard 4 panel roll up door and fuse the top 2 panels together with angle iron, and fuse the bottom 2 panels together, to make the door fold in the middle. Then fabricate the wheel brackets, counterweight pulley system, and latch.