If it's one of those fizzy tablets you put into water then magnesium carbonate and potassium carbonate work too as long as they contain citric acid. They will react (the fizzing) and create the citrate.
Another source of potassium is "low sodium salt". This is often the cheapest form of potassium you can easily get.
Another source of potassium is "low sodium salt". This is often the cheapest form of potassium you can easily get.