Armor Costumes can be obtained from the in-game Cash Shop, and can also be claimed through the trade broker. They are character-bound after the first use, but can be sold if they are unused. Costumes change only the appearance of your character's gear, and do not affect your stats. There are plenty of costumes available in the world of Arborea.
Halloween Costumes
Devlish Succubus Costumes ( Female only )
Titan Force Costumes ( Male only )
Witch Costume ( Elin only )
Steampunk Costumes
Schoolgirl Uniforms
Swimsuits 2013
Pirate Costumes
Shimmering Cloth/Leather/Metal Skin
Cat Costume
Swimsuits
Snowsuits
Black Magic
Kill BAM!
Maid Costumes
References
http://tera.enmasse.com/game-guide/store-items/costumes
http://tera.enmasse.com/game-guide/store-items/loot-boxes
http://tera.enmasse.com/game-guide/store-items/costumes#maid-to-work