If anybody's interested, I found an interesting procedure for manufacturing potassium nitrate -
I tried it, and it really works. Using a $2 box of Kroger Instant Cold Packs (8.6 oz ammonium nitrate) plus 21 oz water and 8 oz of potassium chloride (water softener salt), I got 4.7 oz of raw potassium nitrate.
I don't know how pure it is. So far, I tested it by mixing it 50/50 with table sugar and putting a match to it - it burned very nicely, just like the store-bought potassium nitrate.