The most common source of sulfur is brimstone, a bind of approximately 90% sulfur, 5% quartz and 5% of other impurities, mainly stones and salts. This bind renders the salt insoluble. Brimstone occurs through most of the world and even in Sideral Space.
Other most common haloliths and minerals are stibnite (with antimony), cinnabar (with mercury), pyrite (with iron), gypsum (with calcium and water), galena (with lead) and sulfogemma (with quartz).
Sulfur can also be found in saturated brine in the ocean, specially near volcanoes. Some bacteria even secrete hydrated sulfuric acid within caves. It also occurs in most living beings, and can be extracted from bird eggs.
Sulfur is very useful in an alchemical lab. Its most common use is in the creation of sulfuric acid, itself a precursor to many other chemicals.
Sulfur can be used to create a variety of products including detergents, cellophane, black powder, poisons, fire-related potions and fertilizers.