One available farm: Ancient Kappa can only be found in a small part of the Undercity, just east of where Nicholas is standing this week (see map). There are 8 easily accessible groups of kappa: 7 groups of 4–5 kappa, plus the boss group containing Baubao Wavewrath and 6 other kappa. A ninth group is located at the eastern end of the cross-corridor where the path ends on the farming map, but the close proximity of enemies on the upper level makes it difficult and inefficient to include them in a farming run.
The kappa are hostile to both the Am Fah and the Afflicted, and the last couple groups in the northerly passage will almost always start fighting a group of Afflicted before you can reach them. You won't get any drops from a foe if you weren't actually engaged with it, so this is one time where it is not advisable to let the monsters kill each other off.
Non-Canthan characters generally do not have access to Vizunah Local and should head out from The Marketplace or Nahpui Quarter (location) through Wajjun Bazaar. Some players prefer this route if the Am Fah in Wajjun Bazaar are friendly (e.g. from Seek out Brother Tosai or related quests); the route to the Undercity is free of enemies and only a handful of Am Fah guard the path to the Kappa.