Hi, ! I am not personally aware of a rheumatologist in California that specializes in seronegative RA, but you can start with this search tool and see if it gives you any leads -- https://my.rheumatology.org/find-a-rheumatologist.
You may now that it can sometimes be tricky to find a rheumatologist that treats seronegative RA as seriously as seropositive RA, so I do hope you find a great doctor that offers you support and treatment options that work for you!