Costumes are items that are only equippable in Wardrobe slots. All callings can use any costume in their Wardrobe slots, even if the costume appears to be outside of their normal armor class (i.e. even mages can wear costumes that have the appearance of plate armor).
Costume items can be purchased from Clothier vendors in Meridian and Sanctum. Currently the only items available on these merchants are costume versions of the starting armors for all callings.
It has been said that more costumes will make their way into the game in the future. The possibility of faction-specific and zone-themed sets have also been mentioned.