Poison Gland can be obtained from a mushroom located in the Choir, outside the room with the Brazen Bull. The mushroom containing the Poison Gland can be broken open in many ways, including using the Hammer and Chipper on it, throwing a rock at it, jumping on it, or just clicking on it repeatedly.
If it is not acquired prior to getting captured, it can be obtained in the Nave, in front of the rocks blocking the passage.
