You can find two of these and both unlock the second floor door in Cabin A where you can acquire the Shotgun. One can be found in the Sock inside the Tan Backpack on the Eastern side of the lake. To obtain the key you can use the Guitar, Saucepan, Axe, Kitchen Knife, or even throw the Lantern on it.
The other method is bit more interesting. First head for Cabin 2. You'll need to make at least two of your characters strip down and have sex on the bed to knock the key off of the shelf.