A COMPLETE guide of all 200+ collectibles with timelinks can be found HERE
The collectibles, including 125+ "media" (computer emails, audio files, video files, etc...) are found within all Acts of the game. They can also be seen by using time vision in most instances. Upon picking up the piece of media source, you will be rewarded with the achievement.
You can replay chapters using the timeline at any point, but keep in mind that changing your choices within the Junctions will add new collectibles for you to collect, bringing your percentage down. NOTE:
There are some uniquely titled collectibles depending on your Junction choices, but all collectibles should be in more or less the exact same place; but will have different contents. You can quit to the main menu once you collect them and reach a checkpoint save.
Lastly, use the menu to check "Collectibles" where it'll show your collectibles for each Act/Part so you can easily keep progress.
Alternatively, please see this video which is the beginning to the collectibles guide video playlist:
Upon finding all collectibles, you will have the following achievements:
The Media Baron achievement in Quantum Break worth 49 pointsYou need to find all Emails, presentations, TV Shows, Radios And Posters