You mine sulfur out of the ground. The easiest way to find it is by smell. the stuff shows up in wells with a REALLY bad odor. most often they are salt springs as well.
Its a nice pretty yellow powder.
Potasium nitrate is made by pouring this morning's urine over last nights ashes, turning a bowl over the slush, and allowing it to evaporate. At the end of the day, turn the bowl right side up and collect the crystals that have formed on the surface (use a pottery bowl!). the crystals are the salt peter you seek.
have you become amazed at the ingenuity of the alchemist yet? I mean, who in their right mind would have thought to do that?