Arcane / Aluneth Hidden AppearanceThe Hidden Appearance for Aluneth is Woolmancer's Charge, which is unlocked by the item The Woolmancer's Charge. There is still no known source of this item, but it will likely be discovered soon.
Fire / Felo'melorn Hidden AppearanceThe Hidden Appearance for Felo'melorn is The Star's Design, which is unlocked by in item with the same name. There is still no known source of this item either. It was reported to drop from random mobs in Suramar and Azsuna in the beta, but this has obviously changed in the live version.
Frost / Ebonchill Hidden AppearanceThe Hidden Appearance for Ebonchill is Frostfire Remembrance, which is unlocked by the item Everburning Crystal.
In order to get this item, you need to pick the Teleportation Nexus Class Order Hall Talent. After this, you just need a good amount of luck. Every day you enter the stairs in the Class Hall, you have a small chance to get an emote that tells you that you hear something coming from the portal downstairs. This means that you were lucky, and now you can use any of the five portals, because these will now teleport you to Frostfire Ridge. Here you can talk to the NPC that stands there, and you will receive the Everburning Crystal. One this is done, you can return to your Order Hall and use the teleporters the regular way again.