You could just go for plain tomato juice, which has plenty of potassium and sodium in it, and can tolerate a little more being added if need be. That's if you can handle the acidity, which I can't.
I simply buy low-sodium salt (half sodium chloride, half potassium chloride) from the supermarket and add 1/2tsp to my 27oz/800ml water bottle. I try to get through about 2.5l (just over three bottles' worth) of this a day. It's a basic electrolyte drink, with far more potassium than the commercial ones, and none of the sweeteners or other rubbish. I probably have POTS, though, and need the sodium. I'm sure you could do the same with no-sodium salt if you wanted to, though I'm not quite sure how it would taste. Low-sodium salt tastes weird on food, but I find that diluted in water in this way, it's actually quite pleasant. My friends occasionally spot me adding salt to my water when I'm out, and stare at me in horror. It doesn't actually taste as strong as people expect - it's nothing like seawater, for instance!
I find it odd, and rather funny, that people spend so much money on things like sports drinks, V8 juice or potassium tablets, when you can just buy 350g of low-sodium salt for £0.89, or tomato juice instead of V8 juice if desired. We're spending enough on supplements as it is, after all. Plus I really don't like the idea of drinking lots of sweetener/sugar, colourings and the like. Those sports drinks are marketed as electrolyte drinks, but apart from the sodium, any other electrolytes in there are in insignificant quantities, such as 60mg potassium. I did once experiment with adding magnesium citrate powder to my electrolyte drink, but found that it tasted unpleasant, so I'm using a tablet for the magnesium instead.