My answer to this in my Central California setting I've been using is having a right nasty bunch of bastards in the old state prison at Folsom who have managed to assemble the expertise and the equipment for some limited biodiesel refining. Fuel trickles out from the prison, they sit in the strongest fortress in the region, and everybody's forced to cope with them, no matter how little they're liked. It makes for an interesting part of the landscape, I've found, especially since they have close ties to the most mobile raiders in the area...