Some of: truck stops, larger standard gas stations, campgrounds, larger RV dealerships. I think that of the exits on the outskirts of Tucson on my side of town, 1/2 of them have propane within 1 mile or less.
Check with each campground you will be staying at. Some have it, some may be able to have it delivered, and I would think most could tell you where to get it in the area.
Check the yellow pages for propane dealers. Especially in rural areas. They always have a tank at their office/shop for filling small bottles or tanks. Usually about $20 - $25 for 8 to 10 gallons here in Texas.