I am in San Diego, and everything is decomposed granite here! East County specifically. There is the local river and lake, but you're not allowed to dig up clay at either (it's all either a park or protected wildlife area). There are drainage streams everywhere for when it DOES rain, of course, but those are all on private property and tiny. However, we are absolutely sold on cob, and determined to find a way. We are starting with a small shed to practice.
So, with no local alternative available without being sneaky (and I don't prefer that at all), what are our options? Are there places that sell clay by the truckload? Is clay-based dirt used for anything else - as in, would dirt available at rock/block landscaping supplies possibly be high in clay content? Or some other source? I know local pottery supplies sell clay by the barrel but it's VERY expensive, not a possibility on our budget. And I can't afford to get a handful of dirt from every source and send it in to be tested.
I understand the attitude of 'use what you have', but decomposed granite is useful for almost nothing, with no rocks large enough to build anything with. And, besides, we just love cob. So cob it is.
It's all decomposed granite?? Where are you in east county that you can't find clay? It's pretty hard for me to think of a place in San Diego county that doesn't have clay right under your feet. That's kind of why they made the missions out of adobe; it was right under your feet everywhere you look.
Have you dug down 2 feet, screened the rocks out of the soil, and done a soil test? Failing that, have you talked to excavation companies that need a place to dump fill dirt? If you go around to different excavation sites, you may find someone who is digging up just what you need and would be glad to sell it to you for cheap.
....but it's VERY expensive, not a possibility on our budget. And I can't afford to get a handful of dirt from every source and send it in to be tested...
I would be remiss, even though I want to be encouraging, to not suggest to you that the above quote from you kinda states the reality of working in an earth based material. As a traditionalist, environmentalist, permaculturist, etc. I can not in good faith, propose using materials (or recommend materials) for the bulk of construction that do not have a local source. You are also inhibited by your pecuniary limitations, as you shared them, and the fact that you do face tectonic activities that often counter indicate "pure cob" construction. (Traditional forms are fine and stand well to earthquakes.)
I am glad you love the idea of building with earth, yet it would seem that many have greatly over romanticized this medium, and thereby lose sight of some realities. To conduct "good practice," in architecture the construction process of "means, methods and materials," should (especially with environmentalism in mind) be of the vernacular, natural, and as easy to facilitate as possible. We all ignore these rules (or some of them) at different times. I admit to ignoring them very often as I ship timber frames (especially old ones...) all over the place, but I own the faux pas, and try to make up for it in other ways whenever possible.
I think you will probably find a local source, as already suggested from many different sources. You may even find a contractor close by digging a foundation that has appropriate clays (decompose granite does make up many clay forms) but none of these are going to be either easy or inexpensive. Perhaps you should consider a hybrid? Or better look to the local indigenous cultures and see what they have built with historically. This method will most likely be your prime option, and it may even include some clay...
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!