California is a hugely popular state for drinking Yerba Mate. When I used to travel to San Francisco for work, it was like an Argentinian haven. We would find fernet in bars, yerba mate in cafes, and even yerba mate infused beers. Below are some suggestions if you are looking for Yerba Mate products in California.
Evergreen supermarket is an international grocery store located on Mission Street in San Francisco. They have a fairly large selection of the popular yerba mate brands for you to pick from along with fair prices.
When I visited San Francisco we, were trying to find yerba mate and found Mateveza which is conveniently located across from Dolores park. Mateveza is a yerba mate infused beer and the Mateveza brewery is a small cafe where you can enjoy some beer, empanadas, and other Argentinian treats.