Hi Sahara,
Big welcome to Permies...
....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.
So...
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...
Some positives for your plight:
http://www.sandiegohistory.org/journal/v49-3/adobe.pdf
http://www.sandiegostrawbale.com/index.php?ticrat-adobe-conservation-article,45
http://www.utsandiego.com/uniontrib/20080425/news_1mc25cob.html
Regards,
jay