Dig around for Oral Rehydration recipes online. Gatorade is one commercial variety of course.
Morton’s LiteSalt is a mixture of potassium and sodium, and works well for “rolling yer own”. Don’t try to be a smarty and use Splenda or some sugar-free BS like that, glucose is required for the recipe in order to promote absorption.
Directions: add ingredients separately or mix a large batch together (i.e. the gallon size) and then just measure out the "total" amount listed under 16oz or 1 liter into a water bottle, and then fill the remainder of the 16 oz. or 1 liter bottle with water.