LeBallister’s in Santa Rosa has a wide variety of cover crop seeds, has an online catalogue https://leballisters.com/product-category/cover-crop/ , and does custom blends if you want something other than what they offer in their standard portfolio.  Wilbur-Ellis Co in Healdsburg, and Grow West in Geyserville also sell cover crop seed.  In addition, Oakville Bluegrass Cooperative is up and running, and this fall offered incentives through USDA’s Advancing markets for Producers (AMP) program.  https://www.obc.ag/advancing-markets-for-producers