I found one so far... http://www.walkthrugaragedoors.com
They are based out of Windsor, Ontario Canada, which is OK for me since
I live in Detroit.
However, I am still hoping to find one in the US. I might still try to
modify my own door. If I can purchase a kit at a reasonable price, I
might just by the kit. I think I can see enough of the details in the
picture to design one myself. Fairly simple once you see it. I wonder
why they are not more popular. (Kind of like Murphy beds. I have one
of those too, and I love it, but you just don't see them around here
much.) I am still awating call back from Walk Thru Garage Door on
pricing.