Note: Potassium supplements can be next-to-impossible to find. Seriously, I've tried. In three different states. The internet is your friend.

Also note: Potassium is fat-soluble (which means, among other things, your body does a -terrible- job of regulating its intake, unlike most other vitamins which will pass through your digestive system unabsorbed if you already have sufficient levels), and dangerous to overdose on. There are treatments if it's caught - essentially, rat poison - but they're not particularly pleasant. Be very careful with the stuff.