I needed to clear some patches for the quest but neither the gardening vendor nor the alchemical vendor seem to sell these items. I went to Alchemical vendor because after I failed to find any at the gardening one, I checked the encyclopedia to see that the poison is under the "alchemical powders" category.
I cannot stir the tree poison with my beaker either.
How can I get this one (besides buying it on auction?) how did those people selling poison get it in the first place?