A Ball Gown is displayed on a desk in the fashion shop of Hugo in Venore. This belongs to Santa Claus and can be seen in his house on Vega. Known to exist on Secura, GM Queen of light II is the owner thanks to, what is believed to be, a major bug report. Also exists on Antica, in the hands of some former gamemasters. There is also one on Premia which is currently owned by Azrui. Rumours say its obtainable in the Femor Hills tower quest.
No creatures drop ball gowns.