“Carbonated water.” That's it. Add a little (100%!) juice to make it sweet, if you must!

It's not that expensive to produce carbonated water at home.

If the tap water is too much chlorinated or for other reasons cannot be trusted, one will need a water filter - but in most cases a cheap active-coal filter will suffice.

Then one needs CO2, which can be bought relatively cheaply, and some utility for putting the CO2 into the water, this can also be bought relatively cheaply.

One will drink more water if it's more easily available, and in addition to saving costs one will also have to carry less from the shop.