Dig 2' down and you should find plenty of clay there. I think we found clay only 1-1.5' deep when I helped on a friend's project in SD. Another place to look, is along the side's of roads where the earth has been cut into. I really don't think you'll need to buy any clay. Dig deeper, do a jar test, and practice making a few blocks to see how they hold up and get the mixture right. If you find dry clay, just combine the clay with the sand first, before water. As long as it holds together, it should be sufficient. A cob garden wall I built was only about 15% clay and 85% sand. It's held up just fine, although it wasn't structural or in a seismic zone. Clay's job is really to just hold everything together, so as long as it's not too wet or too dry you should be fine. A few considerations, though. Make sure your walls are thick enough for the earthquakes you WILL get. Though the lack of mortar joints in cob makes it great for zones with seismic activity, no structure is earthquake proof, so I'd still suggest making your structural walls 1.5-2ft thick, curved or with buttresses, and maybe with strategic arches on inner walls or future courtyards. I'm always concerned with earthquakes, so I personally would have support beams, even if some pure cob structures in major seismic zones have held up for centuries. But I am paranoid and tend to overdue things. Anyway, good luck!