There's no single way to do find the best deal other than to shop around. The fastest way might be just to stop somewhere in town, borrow a phone book, and use your cell to call for rates. Sometimes you can do a little negotiation to save some money. I'd also take advantage of your AAA membership and get the various guidebooks, which you can also use to get a ballpark price for
each place.

One other tip is to use the coupon books you'll find at rest stops, truck stops, etc. Those deals are usually only good as walk-up rates, and if they have space available, but I've found some great bargains with those books.