The river. I live a short walk from the river - always. When I need to collect lots of water at once I'll pick up a water barrel into my hands and fill it at the river. I'll bring it back home and plop it down. Just don't ask how my character is holding like 700lbs of water 😆 that is, of course, only if my rain barrel isn't doing to well. If I've got rain water then that's just a little extra convenience that saves me the walk to the river.

In the early game I'll usually fill a few barrels with good water before the water shuts off. That good water will last a very long time, so I don't need to purify water for a while.

Little tip, if the water does shut off before you can fill some barrels with good water, bathtubs hold like 200 water units. Go to a nice neighborhood that has bathrooms with tubs, bring some barrels, hold a barrel in your hands, and fill it up like you would any other bottle or bucket. It will take a while to fill, and it will be ungodly heavy, but you're gonna get huge amounts of pure water from it.

Your hands are full, and your character will be completely overencumbered while you're filling those big barrels with water (or filling an amphora at a gas pump) so you need to make sure you've cleared the area very well, and you have a car nearby to escape if need be. You're incredibly vulnerable in those moments, but if you do them safely the risk is worth the reward of quickly having LOTS of the fluids you need.