Head to Wajjun Bazaar from The Marketplace and go west along the waterfront until you come to Rai Kazu Remedies and Potions. There are a number of other Remedies and Potions collectors scattered throughout Wajjun Bazaar that will make the same trade, if you have the Nahpui Quarter outpost on your map, it is much quicker to go to the trader just north-west of there. They will give you a Plague Remedy for two Naga Pelts, which can be obtained from Naga in Haiju Lagoon.