I have compiled a list of sources for the elements that are available to the amateur chemist. Potassium will be discussed here.
Potassium is a bluish-gray highly reactive alkali metal. It reacts vigorously with water, producing steam, explosions, and purple flames. Potassium forms the superoxide and peroxide when exposed to air, which can be reactive as well. Potassium compounds are vital in the human body. Bananas are well known for their large quantities of potassium. Excess potassium is harmful to the heart.
In element form: No sources found.
In compound form: Potassium chloride is a common salt substitute. It is also used for lethal injections. Potassium hydrogen tartrate is in cream of tartar. Potassium carbonate is made when potassium hydroxide from alkaline batteries is exposed to air.
Here is my sample of potassium. It is potassium carbonate from alkaline and nickel-cadmium batteries. The color is the result of manganese dioxide impurities. This may also contain some bicarbonate.