you get them throughout the main storyline, and they allow you to access storm gates, which lead to other worlds. you get your first one near the end of book 2, your second and third at the beginning of book 4, your fourth at the end of book 4, your fifth at the end of book 10, and your sixth at the beginning of book 13.
They come from quests. As you will have need of them, the quests will come to give you the access you need. Occasionally a promotion quest will send you to a place you do not have access to yet, be patient, you will soon, or you can asker a higher level friend to go there and allow you to teleport to them. If you need them, they will come.