Hi there, Erica!
You'll likely find the most extensive collection of Disney princess costumes in three places: at one of the
Bibbidi Bobbidi Boutique locations, inside World of Disney at Downtown Disney (which is actually the site of one of the Bibbidi Bobbidi Boutiques), and online at
www.DisneyStore.com. The online selection includes items from the retail Disney Stores as well a Parks-specific merchandise.
Officially licensed Disney merchandise is also available at any number of retail outlets, including in-store and online, As Halloween approaches, you should have no trouble finding the princess costume you need!
If you are looking for everyday clothing options, you'll find those online as well.
Please feel free to drop me a line if I can help with a more specific request. I do look forward to helping!
See you later, celebrator,
-Deborah B.