What network CORS RTK service shall I use in California?

Other than using XAG RTK Mobile Station, you can always use network CORS service through local public service or commercial private service

There is a free NTRIP service provided by the California Department of Transportation.

Registration:

https://www.surveymonkey.com/r/CRTN_Registration

How to Connect:

http://sopac-csrc.ucsd.edu/index.php/crtn-connecting/