Potassium nitrate is used as fertilizer. As such it can be bought cheaply and in large quantities. Another possible source for potassium nitrate is 'stump remover', though this is somewhat more expensive. Alternatively, potassium nitrate can be prepared from other chemicals. For example from nitric acid and potassium carbonate or hydroxide (acid-base reaction), or sodium nitrate and potassium chloride (fractional crystallization). Another source of KNO3 is in butcheries (meat stores). They use KNO3 to treat some meats like biltong. At all times, make sure the product is free of acid since this makes the risk of accidental ignition much greater.

The average Joe can synthesize potassium nitrate is by dissolving 40 grams of ammonium nitrate (found in instant cold packs) and 37 grams of potassium chloride (as a sodium chloride salt alternative) in 100mL of water. If the ammonium nitrate is found in the form of prills (or small spheres), it should be dissolved then filtered using a coffee filter. Once the two chemicals are mixed, they need to be heated. Do not boil. As the solution is heating, try to dissolve as much potassium chloride as possible. When done heating, stick it in the refrigerator. What is happening in terms of chemistry is a double displacement reaction is taking place between the potassium chloride and ammonium nitrate producing potassium nitrate and ammonium chloride: KCl + NH₄NO₃ → KNO₃ + NH₄Cl. As the solution cools, the potassium nitrate crystallizes out and leaves ammonium chloride in the solution. Simply remove the crystals and your left with fairly pure KNO₃. See external links for video.

Ace Hardware sells pure potassium nitrate as high yield stump remover.

If your buying your KNO3 as a fertilizer do not buy the prills as you will have to disolve and purify due to binding agents