This stash is found in Grace’s burned home. There are several peggies, including a VIP and flamethrower so dispose of them first so you can concentrate in finding the stash which is inside the residence’s bunker. The bunker’s hatch is inside the house but you have to find your way around the smoldering ruins first to get to it.
After taking out the peggies around the house, read the note first to activate the Prepper stash objective. Next, you have to vault over the open window, right across the prepper note.
Once inside, go through the doorway to the left then turn left again to find a ramp. Go up the ramp, jump back to the ground. Find a table and rack around the corner then jump on it to reach the ledge. Continue and drop down, crawl under the gap to finally reach the bunker’s hatch. Enter the hatch then loot the stash inside.