I can recommend making your own soda with tap water, baking soda (Sodium bicarbonate), and either powdered citric acid or tartaric acid (the former being cheape

I can recommend making your own soda with tap water, baking soda (Sodium bicarbonate), and either powdered citric acid or tartaric acid (the former being cheaper and more available where I live, the latter yielding the slightly-better result IMO). It's basically sherbet powder without the sugar, though nothing keeps you from adding whatever you like to the drink. The result is refreshingly sparkling water. The more soda you add, the less acid the drink becomes, and you can adjust the ratio to taste.