Sulfate of potash, sometimes sold at garden centers.
It's darn near impossible you need it if you dose KNO3, unless you have like 12 Full gown Discus in a 70 gal CO2 enriched tanks with 2.2 w/gal PC lighting tank and feed 2x a day a lot, not likely at all.
You need 4x the N to make K+ limiting than is supplied with KNO3.
So unless 75% of all the N for the plants comes from the tap or fish load, you do not need it.
KCl also works fine for a K+ source.