Depends on what you mean by "northern" California. Brent Walston has Evergreen Gardenworks (
https://www.evergreengardenworks.com/ ) in Kelseyville, CA about 3 hrs north of San Francisco. Mendicino Bonsai (
https://mcbonsai.com/ ) is in Mendicino County on the coast. Both have substantial pre-bonsai. Lone Pine Bonsai (
http://www.lonepinegardens.com/ ) is in Sebastopol, Sonoma County. I'm not as familiar with their material though. Maruyama bonsai (
https://maruyamabonsainursery.wordpress.com/ )is in Sacramento. Lotus Bonsai (
https://www.lotusbonsai.com/ ) is a bit further east.
Of course you can find good material in regular nurseries too.
What county do you live in?