KNO3, like the other dry ferts is an agricultural product, sold in large bags to farmers. You might find large amounts (enough for many years, or to share with an aquarium club) in farm supply stores, or get a cup or so from a farmer.
KNO3 is also sold as a stump remover, but read the label VERY carefully. There are other products that are also stump removers, but are not KNO3.
You could also try hydroponic stores.