The Pharmacy key is a required artifact in the Left Behind DLC chapter Back in a Flash. Ellie takes it from the dead pharmacist at the back of the spore-filled American Princess store, then uses it to open the pharmacy's locked back door.
Ellie's immunity lets her walk through the spores to reach the body. The key opens the room where she spotted a first aid kit, and from there the search leads to the crashed military helicopter.
The first aid kit behind the door turns out to be empty, so Ellie has to look further afield for help for Joel.
Combination note
The Combination note is a required artifact in the Left Behind DLC, found behind the counter of Weston's Pharmacy in the Colorado mall during Back in a Flash. It gives the code for the American Princess store next door, where the key Ellie needs is waiting.
Pharmacist's note
The Pharmacist's note is an artifact in the Left Behind DLC chapter Back in a Flash. It is on the dead pharmacist at the back of the American Princess store; once Ellie takes his key, you can pick up the note too.
Salon note
The Salon note is an artifact in the Left Behind DLC chapter Back in a Flash. It lies next to a dead soldier in the wrecked salon Ellie crosses on her way from the pharmacy to the crashed helicopter, near a health kit.
Atrium note
The Atrium note is an artifact in the Left Behind DLC chapter So Close, set in the present-day Colorado mall. It is in the atrium, to the left of where Joel and Callus are hidden, near the remains of a field surgery.