The availability of Crude Saltpeter at specific outposts changes periodically, so make sure to Skull and Bones Items check the trade posts regularly or visit multiple locations to find it in stock.
Trade Routes for Looting Crude Saltpeter:
The Strait of Herimau and the Gulf of Nakh are key regions where ships carry Crude Saltpeter.
You can also find it further northeast around Rempah's capital, Suny, with ships transporting the material along these routes.
How to Farm Crude Saltpeter
The quickest way to obtain Crude Saltpeter is by boarding ships carrying the material along the trade routes mentioned above. Here are the steps for effectively farming it:

Scout Trade Ships:

Use your spyglass to inspect ships before engaging in combat to Skull and bones items for sale online see if they have Crude Saltpeter in their cargo.

Attacking random ships without checking their cargo may waste time, as the rare material doesn’t drop as frequently when ships are destroyed.