If Civilization goes to shit to such a degree, you shouldn't worry about firearms anymore, but about actually getting shit done. And bows and crossbows get shit done. Guns are only needed if you're in for a really big fight, and even then, you could fuck up a reasonably big enemy force by using traps and ambushes.
BUT I WANT ME GUNZ
Well, in that case you better hoard pdfs on the basics of science and basic chemistry, physics and engineering, because those will come in handy. Tables and charts for specific calculations too. and don't let them stay digital, print everything out, on good quality paper, bind it and keep it dry and cool.
You will need every scrap of knowledge for rebuilding civilization, and therefore, guns.
Sulfur is fucking hard to get, if you don't live near a volcano or a hot spring, you're outta luck, because fucking around with extraction of sulfur from minerals is pretty high on the industrial ladder, and when you manage rig a contraption to condense evaporated sulfur from melting minerals, you may just as well relocate to an area with volcanic activity and work with abundant natural sulfur deposits.
As to saltpetre: stack shit and piss with separating layers of straw, let it rot, and then wash the whole stuff with water, which you then mix with potash, boil it off and let the saltpetre crystallize. done.
That way you get a pure enough product to make a good gunpowder.