All but two of the mushrooms can be found by killing minotaurs around the map. The Fly Agaric aka Red Woodling can be found on the dead minotaur body near the left center of the map. Click on him once and it'll say he died from mushroom poisioning. Click again to raid his inventory. Once you have all but the last mushroom return to Tworsnik and choose option 3 - ask for help finding the mushrooms. He will grudgingly admit he probably already collected all the stomach sickening mushrooms or whatever they're called and give you a key for a chest near the center of the map, north-ish from Tworsnik's camp. Beware the rather challenging Minotaur that spawns when you unlock the chest. Then run back to Tworsnik with the whole batch and get the shield that got you started on this whole agrarian ridiculousness to begin with.