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Medicinal Herbs can be purchased in the village on Mateyo Island or found growing in a few spots scattered throughout the game, while Bizarre Fruits can be bought from Nathan (Nyla's brother) at his pop-up store on the border of Loica (the SE checkpoint that they won't let you through). As always, keep an eye peeled for Nathan's pink hair, which is generally the only hint that you'll get when he's hiding somewhere.
You can't complete Nyla's final quest until after you have busted Nathan out of prison at the Trabunco Checkpoint; for Nyla's final quest, once she gives you the prompt to help her get permission for her family to sell Hunter gear, you just need to advance the Hunter's Guild storyline by doing quests for the captains in each of the Guilds (Coluna, Acclapata Fortress, New Panavinto, Pedregal Springs). Eventually, the Captain in Pedregal Springs will clue you in to a leader of the Hunter's Guild who is doing inspections of the Guild branches. Find him at the Acclapata Fortress branch, and talk to him. If you have completed enough "Bounty Board" quests (item fetch quests and rare monster hunts) throughout the country then he will grant you permission . Go back, talk to Nyla, and the quest is complete.