I was recently made aware of this problem in southern California. There are some alternatives, although I'm not sure of else is banned there.

Kerosene has been mentioned. I'm not the biggest fan since it doesn't evaporate as cleanly but it can work.

I'm not sure if California's ban is on "mineral spirits" or "paint thinner" or both. If not, paint thinner is the same stuff. White gas for camp stoves is similar enough to work, although you should be more careful handling it than you would mineral spirits because stove fuel has more aromatics in it which have more long term toxicity associated with its use. Wear gloves when handling most any organic solvent but be especially careful with "white gas". Lighter fluid is also similar enough to be used.

Finish Line makes Speed Degreaser which may be available but it is expensive and somewhat wasteful due to its aerosol nature.

As far as I understand, the ban on mineral spirits and/or paint thinner is only in southern California/Los Angeles area. If you go outside that area, you might be able to bring some back (I wouldn't suggest putting it on a plane).

Hope this helps.