One is in the stacked wooden crates by the left side of the store.
One is on the bottom of the cupboard in the back of the store behind the rake and broom.
One is inside an Average locked desk. A Lockpick of 50 is needed to open it, or find one of two keys. One key is inside a toolbox and the other one is inside the cash register on the counter in the front of the store.
The Courier may have to fight some low leveled creatures in and outside the store or inside the Ranger station, but overall this is a low level encounter mission.
Search the Zion Ranger station in the north-west of Cueva Guarache (just north of the General Store) for the medical supply kit. The kit itself is sitting on a table in the back room of the station and requires a Medicine skill of 30 to interact with, plus either one roll of duct tape and one bottle of turpentine (the duct tape can be found on a shelf next to the mantle with teddy bears on it and turpentine can be found on the table next to the fridge) to replace contaminated parts, and one bottle of scotch, vodka, or whiskey to sterilize it (all three of which can be found in a nearby fridge), or the Courier can simply salvage the remaining parts of the kit without using any additional items.
Search the Zion Ranger station for a medical supply kit.
Find turpentine for the medical supply kit.
Find duct tape for the medical supply kit.
Salvage the medical supply kit at a campfire.
Optional: Find whiskey, vodka, or scotch to sterilize the medical supply kit.
Find the key to the desk in the Zion General Store.
All of the items required to fix the medical kit can be found within the General Store. The items necessary to fix the medical kit will not spawn until the quest is given by Joshua Graham.
Behind the scenes
A tourist trap is a colloquial term for an establishment, or a group of establishments, that were created with the aim of attracting tourists for their money. Tourist traps will typically provide services, entertainment, food, souvenirs and other poor quality products for tourists to purchase at inflated prices.
The cabinet one of the lunchbox is stored in may have spawned closed and inaccessible instead of broken and open, rendering the quest uncompletable. [verified]
Some of the lunchboxes might also spawn partially stuck in the stores floor or shelf. [verified]