On this page of our guide to Psychonauts 2, you will find Supply Chest Keys available in the Questionable Area.
There are 4 Supply Chest Keys in this level.
Location: The Forgetful Forest.
You will come across this key by jumping through the trees in the Forgetful Forest. Reaching it is easy. You just have to follow the path, i.e. jump on the branches and bounce off the leaves.
This key is on a small platform by a tree. You can reach it by jumping off the bonfire. The air current will carry you and all you need to do is fly towards the correct location.
Location: Fierro's Funicular.
You can obtain this key as soon as you enter the top part of this location. You need to enter the mist-covered area, near the rocks you will see a glowing key.
Location: Sassclops cave.
To get to this key, you have to climb the stalactites and jump between them until you reach the stone platform with the key.