Butterick 5174 is a Halloween costume for Princess Leia, but you may have trouble finding it as it is no longer in print.

Simplicity 6673 is another pattern you can use and alter.

To make the hood, you can use Simplicty 9887 or make your own pattern using this diagram:

The dress should be made from white polyester that is thick enough that you can't see through. The patterns can either be found locally or at online pattern shops like MonsterPatterns.com.