Got a friend who runs cattle on a farm? If so, chances are that he's got a manure-pile. Dig around in the bottom of a good OLD manure-pile and you'll come across levels that appear WET and look greasy. Keep going just a bit and you'll start finding what look like lumps of wet maple sugar. This is crude saltpeter. Bring these lumps out and chuck them onto a tarp or something to dry, or just pop them into a 10-gallon drum.
When you have all you want, put the manure-pile back together and wander off with your barrel. Add hot water to the barrel JUST sufficient to dissolve all your crude nitre. Give it a stir. Yeah, it stinks, but it's sort of a CLEAN stink: not like Government at all.
Now start heating your nice barrel, watching the steam boiling itself off. The stuff is now becoming a superconcentrated solution. Make yourself up a couple of coat-hangers into sort-of tree forms and when the stuff begins trying to crystallise out on the sides of your barrel, hang these in and let the pretty white crystals form around your hangers. Later, as the level gets down, the stuff will start crystallising in yellow and then light brown and finally in a darker brown. The lighter the colour is, the purer the stuff is.
Ideally, you will just want the white, but there's nothing to stop you from taking the yellow and dissolving it and recrystallising it out, raising its purity as you do so. You powder it before you use it. And DON'T eat any! You can purify ALL of it by adding stages, even the stinky brown stuff.
Europe fought its wars like this for 300 years before the rich guano beds were discovered in Chile, then they spent the next 300 years fighting wars on Chile Saltpetre, made exactly the same way, just using bird crap.
Welcome to the Late Middle Ages, friend!
(For a really good read, get a copy of H. Beam Piper's "Lord Kalvan of Otherwhen". It was Piper's last book, unfortunately. Publisher took too long accepting it and Piper shot himself out of sheer depression. If he had gone to the Post Office instead, he would have found the cheque which would have kept him alive. Terrible Tragedy. Great Book. What to do when God controls the munitions industry!)