Two of the best places I have found to get poison loot drops are Everfrost around the South Tower area, and Unrest.
In Everfrost at south tower, you have orcish mountaineers, shamans and troopers who all often drop lixt wing stalks and cyclamine corms. And a large supply of Giant spiders, who seem to drop poison sacs more often than other giant spiders.
In Unrest, I have gotten crystalline marrow, grave mold, that phalloides mushroom thingee (forget full name), unlimited zombie skins for making vials, and embalming dust.