May sound icky, but fermented doggy doo, is very high in potassium, and is great for bananas and tomatoes. I have a bokashi pet poo digester, and this is a great way of processing doggy doo, for the garden.
For those who do not want to use poo another option is to get a bag of sodium free water softener salt, which is potassium chloride. And it is only about $7 for 50 pounds.