A lot of people in the area collect their own. If you collect from public lands you need a permit and you are not supposed to sell. I don't think they care if you sell a tree that you have been maintaining as bonsai for a number of years, but you are definitely not supposed to collect just to put the tree in a grow box and turn around and sell it.
Permits for California juniper are not difficult to obtain. If you go north towards Inyo, you can even get tree cutting permits:
https://www.fs.usda.gov/main/inyo/passes-permits/forestproducts
Many of the local bonsai clubs have relationships with ranchers / private land owners who allow you to collect trees on their properties.
I know this didn't answer your question directly, but I am not aware of a good source that sells California junipers.