You don't find poison extract, it's a recipe. The witch tells you it during the day I think (might be the night if instead it's a question about love potion or aphrodisiacs).
Also you could use the guide as it says the recipe there.
If you study at the witch hut at night you'll see a diagram showing three recipes, for poison extract, antitoxin and poison bomb which you have to work out from the diagram. Alternatively the wiki has a list of recipes if you want to do it the easy way.
