These are very common during your playthrough. And the fact that you only need fifteen is a piece of cake. Recommend to start looking for these as early as possible. If not you will come across plenty of them towards the later chapters. They glow with bright blue light, so you really can't miss them. You will usually find these Armory in a group. They cost 250 Energon credits to open. Inside they will hold all sorts of goods from primary & heavy weapons to different kinds of perks and TECH items.
NOTE: You can farm these by restarting last checkpoint or dying.