I have heard of people using marine batteries and 110-volt invertors to do gigs with... you can find those at WalMart. However it's been my experience that with a little scouting such things are often unnecessary. AC power is often available in places you wouldn't expect - you just have to find it and be good at routing extension cords. Farmer's markets all have power outlets if they have lights... even public parks usually have hidden outlets. If you really have to go for a primitive location, maybe you should get a little Honda generator.