Update -

I was in an art supply store today (in California) and, lo and behold, there was naptha back on the shelf. An employee told me that California art supply stores can now legally sell naptha again due to pressure brought to bear on legislators by art supply retailer lobby group. Evidently, banning it created a hardship as it is a major part of their business as well as selling other types of thinners so they appealed and got permission to sell it in quantities up to one liter I think. Evidently this only applies to art supply stores. So, that's where Californians can buy it. For now, anyway.