What is the best source of adding electrolytes to regular drinking water?

Tap water: Tap water is a good source of regular drinking water. Otherwise buy something like gatorade or if you are talking about someone for example who has diarrhea in the drugstore there are packets of electrolytes called oral rehydration packets that can be added to tap water that are made specifically for rehydration. Just follow the direction for mixing and for age groups. Pedialyte is best for kids.