You can find the tea in Verdistis, in the alchemist's house (near the south west corner of the part). IIRC you might need level 5 Lockpick to open the hatch. Close the casks before moving them around.
BTW, the talking tree is required to get a couple quest items (one for Corinna in Verdistis, the other for the alchemist who lives west of the tree), so don't kill it before getting them.
