The commercial answer to wanting flavoured sparkling water is a can of natural flavours. The real answer is a peach and twenty minutes.
Every flavour here has three separate things people tend to run together: the infusion, which is fruit in water with no sugar; the syrup, which is cooked and sweet and keeps for weeks; and the build, which is the ratio that turns either one into a drink. Plus, for each, the thing people try that doesn't work.
Start with the water: sparkling water worth buying, or syrups already made if you'd rather skip the pan.