It is found on the south end of the Resort Map, about halfway between Bunker 06 and the City Tunnel Gas Station. From the bunker, the player can follow the road marked on the map that would lead them to the gas station or westward. Following the road from the bunker, the player will see a small boat. The player must go past the small boat, into the cave, and exit the other side. There, the player will see a larger capsized boat. Next to the boat will be a rocky cliff with several palm trees growing from it. The skull is at the top of the cliff where the player must jump over a bush onto the rocks to the top. It will be lying in a box next to a metal chest.
It should be noted that it's possible to visit the Supermarket even after the player has completed the quest associated with it. Check the Sewers Map for directions.