Sulfur is two things at once: a Repair Tool component you need very early, and the cheapest Strong Acid alt-recipe ingredient once you can process it with Gold. Both reasons send you to the hydrothermal vents.
Early game — ~200m SE of Lifepod near Coral Crabs (~25m depth): The single most important Sulfur tip. The Coral Gardens biome has a small Sulfur cluster within easy free-dive range, near the Coral Crab habitat about 200m southeast of your Lifepod. This is where you get your first Sulfur for the Repair Tool recipe — no Heat Tolerance Adaptation required.
Karakorum (100-400m): The serious Sulfur farm. Sulfur appears throughout all three Karakorum sub-regions (Observatory, Power Plant, Root Canyons). Combine with Atacamite, Celestine, and Gold runs.
Hydrothermal vent fields: Dense Sulfur in the vent biome, but the heat DOT requires the Heat Tolerance Adaptation or a Heat Resistance Chip. Skip this until mid-game — the surface SE cluster covers your first Repair Tool needs.
For your very first Repair Tool you only need 1 Sulfur. The path of least resistance is the cluster ~200m SE of the Lifepod, ~25m depth, near the Coral Crab habitat. A simple free-dive with a Standard Air Tank gets it. Do not dive into the vents on your first descent — you don't have Heat Tolerance yet and the heat DOT will burn through your medkits.
