On this page of our guide to Psychonauts 2 you can view the locations of Half-A-Minds in Cruller's Correspodence.
There is only a single Half-A-Mind in this level.
Location: International Dead Letter Office.
The Half-A-Mind is at the very bottom of the bookcase. All you have to do is to slide down a blanket of leaves and reach a small ledge. The collectible is at the top of it.