The final place to farm Stigma's at level 20 is the Poison Wing Vespine in Brusthonin. These mobs have a chance to drop both the Healing Conduit for Chanters as well as Flurry for Assassins. The Poison Wing Vespines are quite spread out over the whole area around the Polluted Waste.
This area is probably the worst place to farm for Stigma's out of all the level 20 Stigma Farming areas. I'd actually only recommend you farm here if you need the Chanter or Assassin Stigma or if for some reason it is inflated on your server.
Green Vespine Sap (Vendor Trash)