First check the USDA soil map for you location: https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx
If you want to make adobe or compressed earth blocks you need between 15-30% clay contents in your soil. 25% is theoretically optimal, but it has to be determined empirically.
If you want to make your own mix from scratch then it's convenient to have access to pure clay.
Few years ago when I started building my CEB house in California and I faced the same problem, but within 1 day of internet search and few phone calls I have found a place that sells rocks, gravel, sand and also clay. I have checked it first, found the lab analysis of this particular clay and purchased it afterwards.