I usually fill up at publicly accessible spigots and convenience store soda fountains.

I'll note that I ran across a couple of small villages in New Mexico and Colorado where the municipal water supply was unfit for drinking and had to buy water.

Other than that, I've filtered water from streams, begged it from folks working in their yards, used the hose spigots on churches and city halls and filled up in restaurant kitchens. I even found an almost-full 1 liter bottle on the side of the road on a hot day, 10 miles after I'd run my bottles dry. I said a little prayer of sincere thanks as I downed it. Best. Water. Ever.

One caution is self-treating and drinking water from streams near golf courses and agricultural areas. I fear that these sources may have really high concentrations of fertilizers and pesticides. My prefilter and Steripen would have no effect on this, so I avoid them.