By foundation I mean the trailer...
Like the Tiny House Plans, I chose piece of mind over budget when deciding on a trailer. Instead of fumbling around Craigslist or whatever, buying a cheap, used trailer and trying to make it suitable to build a house on, I just called up a local custom trailer dealer and had them handle everything.
Fortunately I had made a relationship through work with the trailer dealer I bought from. These guys fabricated and erected the structural steel for a commercial building that I managed last year and I just happened to ask if they built trailers as well. As it turns out, they are just a dealer that has another fabricator build for them, but it was still a working relationship, they have a lot of experience with tiny house customers, and they were really easy to work with. It was a win-win as far as I was concerned.
The plans that I bought from TinyHousePlans.com have a detail drawing and general info for a trailer. All I did was send this detail and info over to Ribolt and tell them I wanted it 22' instead of 24' and that was that. The trailer was done in less than one week. A check for them, a trailer title for me, a simple swap of the hitch for a bigger ball and the old F150 was ready to tow that bad boy to the build site.
If you're within reasonable drive to Maysville, KY (technically Tollesboro), I highly recommend calling up Ribolt Fabrication. They deal with tiny house customers regularly so just turn them loose and pick it up when it's finished.