John, diphantoine is seldom prescribed any more for MC. Mexiletine is the preferred medication, and some of the others are acetazolamide, carbamazepine, lamotrigene and flecainide. You might want to have your pharmacist see if there is any type of program for the Mexitil to help cover the payment, and if not maybe you can file an appeal.
Potassium is in all fruits and vegetables. It's probably highest in bananas. But too much can worsen myotonia, so just try smaller amounts at first if you use a supplement.
I would give the Acetyl-L-Carnitine a try. It really helped my muscle strength and stamina. This is a link to Amazon.uk: Acetyl-L-Carnitine