Steve, I wasn't trying to be funny. Here are my thoughts.
If you really have to have a prefab steel building:
1. Pay the company that you want to buy your building from, buy the engineering plans and calcs. Ask if they can include the foundation design too. If not, then hire an architect or civil engineer or structural engineer to design the foundation system. This is what you need to submit for permit along with a site plan etc.
If you can live with a wood framed building:
2. Seek out a local drafting service to draw plans for you. In California, one does not need to be an architect or and engineer to design and build a wood framed residential structure. See the link California Residential Code 2019 based on the International Residential Code 2018 (IRC 2018) (up.codes) Ok, I'm sure the California Residential Code 2019 is probably above your head but a good drafting service knows how to do this and they usually specialize in small projects. Another up side to wood framing is you can most likely find more workers that know how to do this than steel.
Just my 2-cents