I found potassium sulfate at the nursery a long time ago but the same nursery doesn't carry it any longer. You would go broke fast using a salt substitute for K. Isn't some water softener potassium chloride? Be careful dosing the Fleet, it doesn't look like anything but has plenty of phosphorus in it.
Plaster of paris is a good source for calcium and baking soda is carbonate but unfortunately also sodium. Epsom salt is magnesium sulfate plus a lot of water.
And read the labels extremely carefully and research the exact brand online to avoid unpleasant surprises.