Costumes are only one way to change your appearances. You can also use tonics (wich are in game rewards, some are single usage ones, others are ever lasting, meaning you can change to that forum as much as you want).
A costume gives your character a special look wich is unique and can only be bought in the store. All costumes ever brought out are still available (afaik). in many causes however you keep your own facial and sometimes even your hairstyle.
A tonic changes you into a total new model. The model depends on the kind of tonic. So there are tonics that change you into a margonite or a snowman, but also one that changes you into a hero like Jora or Gwen.
Last of all there are armor-sets. those are your actually fighting close wich can have different looks and feels.
If a character has a look you like (and is in an outpost) you can just whisper them. Most people are doing it to show it of and are therefore happy to explain to you what it is they did (costume, armor or tonic) to look that way.