Back 40 has at least a few options. Many of the trailheads/parking lots have water fountains with bottle fill stations. Unfortunately not all of them are operational. Two weeks ago the Lake Ann trailhead at the end of Flo Ride was off for some reason. But the most remote and northern one (most important one!) was on! This lot:
But probably many more. And there's a convenience store a few miles off the north side as well, with a sign directing you ro it. And hell, if nothing else you're around a shit-ton of houses. Lots of water hoses if you don't mind asking.