[dice]

Molasses at a tablespoon per gallon of water, every two weeks, is another
fast potassium fix. Molasses is 5% potassium and breaks down quickly.

Use the darkest, thickest stuff you can find. Mix the tablespoon into
a quart of hot water, stir it up until it is evenly mixed, pour in a gallon
jug, then fill up the rest of the way with cool water. Ready to use.

edit: (Not suggesting the other alternatives will not work as
potassium sources, just that this one will. What are your sources
of other nutrients? Tomatoes need more than just potassium.)