At the start of the mission, in the room across the stairs you can find the first Non Believer Poster.
Non Believer no.2
Go up the stairs, and follow the path through a small air duck shaft until you reach a room with generators and lanterns on the floor. Use the lanterns to incinerate the Non Believer poster.
Non Believer no.3
When you come to the point where you need to break the window in order to proceed with the main mission, after entering the next room, check the wall to the right, and you will find a Non Believer poster.
Non Believer no.4
The next Non Believer Poster is located in a small room right after Lara obtains assault rifle.
Non Believer no.5
The last Non Believer Poster is in the room with broken radio where Lara attempts to broadcast SOS signal. Non Believer Poster is adjacent to the smashed door.