We used to do this in high school and bought a single large bag of potassium nitrate from a local commercial agriculture shop. We even called ahead to make sure

We used to do this in high school and bought a single large bag of potassium nitrate from a local commercial agriculture shop. We even called ahead to make sure that it had specific chemical designations on the bag because it was far enough away to be a pain to get to. Looking back it was very dumb in a legal sense (and a lung health sense haha).