In the U.S., you could purchase saltpeter over the counter, at any pharmacy. Don’t know if that is still the case. Have not made gun powder since I was a teen.
If you have patience and somewhere to hide the smell you.can use human urine or animal urine to make a niter bed let it ferment with natural airborne bacteria for 6 months then wash that bed with woodash water. The effluent will crystallize as it evaporates in a tray, those crystals will be nearly pure potassium nitrate. This is how it was made for thousands of years before commercial sized deposits were found in the Chilean desert and later modern chemical synthesis. It’s not ha rd people in BC times did it.