You can find this quest in Mother’s Rise. Speak to Fia to start it. Once the conversation is over follow your objective markers to the two points on the Brave Trails and check the two caches to find them empty. Head back to Fia and let her know, and she’ll tell you to go speak to Jun at his mountaintop hut. Follow your quest marker until you get there and you’ll find Jun locked in his hut. Speak to him then head after the Outcasts who stole the medicine by using your Focus to follow their tracks. When you find them they’ll be under attack from a Sawtooth and a couple of Watchers. Hit the Sawtooth’s Blaze canister with a fire arrow, then set up tripcaster wires to catch it as it tries to them come after you.
When it and the Watchers are down, speak to the leader of the Outcasts, then go back and speak to Jun to complete the quest.