Kill the Naga Raincallers/Bone Collectors/Warriors in Panjiang Peninsula or Haiju Lagoon...or try "WTB" at The Marketplace or Kaineng...just be sure it's pelts and not hides you are buying
2 or 3 quick runs thru Haiju Lagoon should get you the pelts you need. Just head for the beach (from Zen Daijun) and take all the Naga out - avoid the base in the centre by going around it - and its a fast run.
Otherwise try Zen Daijun and Seitung Harbour with "Looking for Naga Pelts" - someone will have some for you to buy.