The only thing I can get myself to drink is flavoured water, can I make this less expensive?

Feel free to remove if not allowed i wasn't sure where else to ask. I'm titrating Elvanse right now and doing well (methylphenidate felt like I was dying). Irritating side effect, im super thirsty, I've never liked plain water and I hate it more now, the whole vitamin C thing keeps me away from juice and im not really into fizzy drinks.

So the only way I can keep up hydration is flavoured water without added vitamins, however this bottled water thing is turning out quite expensive and im on disability because a bunch of other health problems. I did do a quick Google but all I can find available is like squash syrups which are not what I want as I often feel more thirsty after them. Essentially I want whatever flavouring they use to make flavoured water. Syrup, powder idc. Anyone know where I can get this?