Things to consider when placing a settlement.

[Redrobes]

The manufacture of such items would be very localized but you can transport those items so could still potentially be available where the resources are not directly placed for the manufacture of them. As Waldronate says, there is diminishing odds the farther away you get but I think for those items like black powder or ammo its more about the cost. I guess the smaller, more advanced and expensive items have a bigger radius for sale even if their rarity or sheer expense to buy is still a big problem. Also if an item itself is rare or expensive then its transportation would then be more expensive too due to risks and protection required.

Also, I can add that nitrates come from pig farms, charcoal grows on trees. Only the sulphur would be a bit of a problem and generally, although steel is expensive, its ubiquitous in fantasy role playing games. The knowledge of the process to create black powder would be rare though. I would imagine that those who knew it would keep it to themselves.