If you're a non-resident visitor, you probably need to buy travel health insurance from your country of origin. U.S. health insurance will typically only sell to residents. You don't have to be permanent resident, but do have to be a legal resident of the state in which you're buying health insurance.

For example, coming from Denmark, I was able to buy travel health insurance for coverage in the U.S., from http://www.gouda.dk/, for $100/mo. It would've been cheaper if it were for travel to any other country besides the U.S. (there are three price categories: Europe, world excl. US, world incl. US), but c'est la vie.

If you become a California resident, you can buy health insurance here: https://www.coveredca.com/