I'll second the pedialyte - it is the only thing that is widely available that is physiologically correct in terms of what it pushes and pulls osmotically. The generic versions are the same, and you can buy it at any grocery, drug or discount store.

There are some things like elete that can be mixed with water to achieve the same thing, but I find that those are sometimes hard to come by. My LBS doesn't always have it. AND if they arent mixed with the right amount of fluid then you don't get the osmotic results you want.

Your rehydration solution needs to osmotically PUSH fluid into your body - not pull it out. Gatorade and other sweet drinks pull it out and actually dehydrate you. Pedialyte does exactly what it is supposed to do.