The best way is to make an electrolyte drink yourself. Here's my recipe:
- 2 Liters of water
- 1/2 teaspoon of pink salt
- 1 teaspoon of potassium chloride
- 1/4 teaspoon of food grade epsom salt
- Lemon and stevia used sparingly to taste
It's not the best tasting thing ever, but you get used to it. Using filtered water will make it much more palatable. I think salts make poor tap water quality more obvious.
You can always drink Powerade Zero or something like that. I don't think there's really anything wrong with that, and I drink those sometimes. It's just more expensive and the artificial sweetener might spike insulin.(I doubt that's a big deal) The recipe I described is pretty dirt cheap.
I used to have a hard time fasting for more than a day or doing a ketogenic diet because I simply wasn't getting my electrolytes. This was accelerated by the fact that I was drinking way too much water and urinating them out. Replacing electrolytes throughout the day keeps me from feeling miserable.