It's a bit of a grind this one but depending on how many you find during your play through you can get this done in an hour or less.
Leave it until you have finished all side quests as some of these will only become available in certain areas of the game. Follow these three videos to collect the ones you missed whilst playing:
http://www.playstationtrophies.org/game/dying-light-ps4/trop...Go into your inventory, hit
and it will take you to your collectibles page. Go through all the ones your missing and then the videos conveniently list the times in the video so you can find them easier.
Fastest way is probably to do what I did and get all the collectibles in one area then travel to the next to get all the ones there rather than going back and forth. All credit to www.playstationtrophies.org for the video.