Is sulfur necessary?
You can still make a respectable gunpowder without sulfur.
The purpose of sulfur is twofold (iirc)
- Lowering the ignition temperature of the powder
- A power "boost" in that sulfur burns as well.
- You can just leave out the sulfur. it will be anemic so probably more powder is needed per shot. This is because more energy per chemical reaction is eaten to fully ignite the powder. (mental aid: diesel itself is hard to light, but mix in gasoline and it goes off like crazy)
- You can substitute sulfur for something else that burns hot and is easy to light. I heard mention of sugar, alcohols(dubious), iron rust(depends on the type of rust), aluminum powder(careful), oils(easy to light stuff like wd-40,gas,etc), etc. you'll need to experiment on your own which will work for you.
Frame challenge 2.
Is black powder weaponry necessary?
There are other methods to get to a "gun"
- PCP guns (air guns) can be hand pumped. guns like the Airforce Texan are plenty lethal. It still has the problem of finding bullets to shoot but that's a shared problem with the BP gun variety.
- A spring piston air rifle may not be as deadly but if you have oil or some other easily flammable substance you can use a method called dieseling. this is where you deliberately put in fuel behind the bullet where the piston of the air gun will detonate it like a diesel engine does. Air guns are not designed for this so this will shorten your gun's useful life.
- Bows, crossbows, etc. you can still learn and practice to make traditional weaponry. They are still plenty lethal and resources are availiable. (skill requirement)
Finally, answering your question (somewhat) volcanoes, hotsprings, geysers, etc. to find the ore types you'll need a good eye for geology and refining know-how.
Hail mary (aka unrealistic af) is that you load up on onions, eggs, etc. eat it, and try to capture your farts alongside your sewer gas from collecting waste. you can then try to implement your own version of a claus reactor. (stinky farts have h2s, and so does sewer gas, claus reactor converts that to elemental suflur)