I look for (liquid gas) fuel in quart bottles either at Wal-Mart, camping stores or hunting fishing stores as I travel or will try to buy it from someone at the campgrounds. Otherwise, I buy a quart at the gas station.
It is rare now days to find liquid gas from someone in a campground. Quart cans of liquid gas in stores are also rare. Camping stores that cater to backpackers that sell liquid gas by the ounce are apt to charge a very high price. Expect to pay as much for a quart as you'd pay for a gallon at a big box store. Rather than pay so much per ounce, I bought a gallon can, filled my bottles and left rest at bike shop, saying give to next touring cyclist.