A few of the Exotics in Destiny 2 are tied to specific quests.
These are obtained from side quests after beating the story.
Exotic Side Quests
Some are unlock from simply playing the story.
Main Mission: Riptide
Main Mission: Sacrilege
- Graviton Lance (Pulse Rifle)
Everything else not listed here is from random drops, winning Crucible Matches, Strikes, Nightfalls, Heroic Public Events, Raids, or buying them from Xur.