Shrunk ogleroot can be found in the witch's house in Taverley by playing the piano, as part of the Grim Tales quest.
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.
For an exhaustive list of all known sources for this item, see here