Are you sure you're consuming enough potassium to require it to be on a doser? Potassium can be consumed in reef tanks, but needing to add it so frequently that it has to be on a doser sounds like testing error.
Regardless, you can use potassium chloride powder to make your own stock solution. I would choose a powder that is at least food-grade,
like this one from Etsy.
You could make a stock solution of any strength. I personally would shoot for something generic, like 1 ml per 10 gallons gives a 1 ppm increase in K. To do this, mix about 72.187g of KCl in 1 liter of water. KCl is about 52.44% K by weight. So, 72.187 g in 1 liter of water will give you about 37.854 g/L of K, which is about 37,854 ppm. 1 mL of this solution should raise 10g of tank water by 1 ppm. To make one gallon of this solution, add 280g of the KCl to one gallon of water.
It looks like from your other posts that your tank is 165 gallons. To calculate doses of this supplement, multiply the ppm increase you want by 16.5. Let's say you want a 10 ppm increase of K. You would need to dose 165 ml (16.5 * 10 = 165 mL).
Randy or anyone else, feel free to double check my math
