RV dump stations (Flying J, some other truck stops, RV parks, state parks, some Camping World stores) are a water source, as are the faucets at truck stop fuel islands.
At fuel islands, one of us can usually fill the water tanks while the other is fueling, so there is no delay for any drivers that may be waiting in line.
Generally, you can get water anywhere you can find a garden hose spout and permission to use it. We carry two lengths of 25 foot garden hose (50 feet total) in the truck, which is usaually enough. At relatives' homes, we sometimes have to ask to use their longer hoses to reach from their house to the truck.
I know one driver who would slip the guys at Speedco a couple bucks extra for letting them use the hose in the store while they were getting their oil changed. He would do that in a pinch in the winter, when working water faucets were harder to find.
At Volvo dealerships, when we have had the truck in the shop for work, I have refilled our water tanks using their faucets in the shop. Never had to slip the guys any extra money. Just asked and received an easy yes.
Generally, we do not need to get creative in finding water. Flying J dump stations and faucets at truck stop fuel islands usually meet our needs.