In RuneScape how do you get an costume

It depends on the costume. For holiday costume items, for holidays like Christmas, Easter, and Halloween, Jagex (the company that runs Runescape) hosts events, which are like mini-quests. When you complete the quest, you are given a piece of a costume.

Some holiday items from past events are available on the Grand Exchange, but for huge sums of Runescape GP (tens of millions)

Other costumes you can earn points for after successfully completing a random event, if you choose the "Save up for outfit!" option after opening the random event gift. When you save up enough points, you can redeem certain costumes (not the ones from holiday events) at Thessalia's Fine Clothes near the center of Varrock.


Still more costumes are given as a reward in Balthazar Beauregard's Big Top Bonanza. The higher your level in either Ranged, Magic, or Agility, the better the costume you are given.