This page of The Last of Us game guide contains information on where to find the comic books in the Bus Depot chapter. Those items are collectibles-secrets. Our walkthrough indicates the exact location of each artifact and pendant.
Comic #12 - Precipitate
Once you finish observing the giraffes with Ellie, walk through the nearby door and climb down the stairs.
This way you will reach a big room with men's restroom to the right.
The comic is inside.
Comic #13 - Catalyst
Once you throw down the ladder to Ellie inside the tunnel, turn right.
You will notice a ventilation shaft there. At the end of the shaft, there is the thirteenth comic.