My father owns a construction business, and we keep 10,000 gallons of diesel and 2,000 gallons of gas, plus another couple hundred gallons of waste oil (until it gets pumped out).

You'd be surprised how many other local garages (and farms) have pretty big tanks of fuel. Also, I talk to the fuel delivery guy all the time and I ask him about his route. I've made note of who the closest people with large tanks are. (Just in case.)

By the way, don't forget that any house that uses oil heat is a source of diesel fuel and you can often get access to their tank right from the front lawn.