Off today to find some potash.

I get mine from a Garden Center that sells these fertilizers individually.
Most stores nearby only carry the already mixed type which we don't use in
ponds.

If not, there's a farmer's supply place down the road a piece, and they
might carry it.

Water softener suppliers carry 40 lb bags of potassium chloride for $15.
40 lbs is a hundred year supply for most people...