Hi, Haley!
There are two ways to go about finding the prettiest princess dress for your little one.
The
Disney Store offers a variety of Park merchandise. Their
Halloween boutique is no exception. There does seem to be crossover between what is offered online and what is available at the Parks.
If you are looking for something more elaborate that you actually saw at the Walt Disney World Resort, you might want to contact the Disney Merchandising Division. They are open from Monday - Friday, from 9:00 am - 5:30 PM, Eastern time.
You can contact these Cast Members by phone: toll-free at 877-560-6477 or at 407-363-6200. That might be easiest for you, since you'll be able to speak directly to someone, describe what you saw and where you saw it, and so on. In case you need it, the email address is wdw.mail.order@disneyworld.com.
Good luck in your quest!
See you later, celebrator!
-Deborah B.