Saltpeter, traditionally, was collected by pouring water through outhouse leavings, and then pooling it. The white crystals that form in the water are saltpeter. This is how colonials were able to manufacture their own black powder, by adding sulfer and charcoal. Search the web for recipes - JimRob told me to not mention it again.
LOL! I get it now. Yes, you're absolutely right. Another source of potassium -- potash -- was wood ashes.
By the way, the charcoal does not actually react when gunpowder oxidizes. It simply serves as a medium to sustain the reaction. The sulfur actually oxidizes to sulfur dioxide and hydrogen sulfide, but it too provides a slow-burning combustible that sustains the oxidation of the KNO3, which is the primary fulminating ingredient.
One of the best things -- or the worst -- my parents ever did was buying me that chemistry set ...