Enchanter's Elixir Location

I've been looking for the Enchanter's Elixir everywhere, but it never seems to appear among the items for sale in various shops, nor can I find it around the world or through looting. Is it that rare? I read that there should be a guaranteed spawn in Anise's cabin or at Faldar's Tooth, but I've checked and there are only minor enchanting potions. Is there a certified spawn point? I don't know if it depends on the level or the difficulty, I'm level 75 and playing on Hard