Hi Ashley,
I have a daughter who loves American Girl dolls too! Disney sells a version called My Disney Girl Doll, and there are a variety of costumes available for purchase. World of Disney in Downtown Disney usually has a large selection, but you can find them at other large stores throughout Walt Disney World.
They are not currently available at the Disney Store online, but your local Disney Store may have a selection available. I know my little girl has a Minnie Mouse and Belle costume that she loves.
If you really want to splurge and treat your daughter to a special day,
My Disney Girl's Perfectly Princess Tea at Disney's Grand Floridian Resort is an incredible experience. Your daughter will receive her own My Disney Girl dressed in a beautiful Princess Aurora gown. She'll enjoy tea, stories, special surprises, and a special meet and greet with Princess Aurora or another Disney Princess.
Hope this helps. Have a magical vacation!