If you need another one for Grim Tales, you can kill an Experiment No. 2 down the manhole near the entrance to the witch's house, or pick one up from the pile in the room with the experiments; assuming you've already used a shrink-me-quick potion once before to get into the mouse hole inside the witch's house. This is the second ingredient of shrink-me-quick potion used during Grim Tales quest.