The spot is quite hard to find for first timers.
It's to the right of the stairs that lead to top of the volcano. You'll see a statue (Engraver next to it) and right next to it is 2 trees.. The spot to collect Sulfur is right there. (X:4022 Y:2875)
Kelan Village(Cave)
You can also get Sulfur in the 2nd floor of Kelan Village. Walk to the narrow path and you'll the collecting spot.(X1962 Y:1255)
Compounding[]
Recipe
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Recycling[]
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Fishing[]
You can fish for it with Fishing Primary lvl 5+, or at the cliffs just south of the North Island entrance to the Subway.