The Princess Outfit consists of a princess blouse and princess skirt. It can be acquired by trading a frog token from the Frogs random event to Thessalia of Thessalia's Fine Clothes in Varrock. The outfit cannot be traded to other players or sold to stores. It should be noted that only a female character may wear the princess outfit. If gender is changed whilst wearing it (Makeover Mage) then the outfit will change into a prince costume, the male equivalent, which is obtained in the same way.
If you happen to lose the Princess Outfit, you may not get another in Thessalia's Fine Clothes shop in Varrock. You have to get the random event again.