The collections spawn randomly but in the the same spots. Those are places people have found them but they won't always be there.
They also in most cases give you a random collection piece from that area and that makes it hard to complete. That's why you end up getting the same ones over and over from different spots. Usually one piece in a collection is rare so it can get frustrating.
There are feats for collecting briefings and collections and there are a handful of styles that can only be gotten by completing them.