Castle Couture at the Magic Kingdom has a good selection but I think the best place is the Disney Store at Downtown Disney. The prices are basically the same no matter where you go.
We just got back and the Character Warehouse store at the Orlando Outlets that is further from the park had Tangled and Jasmine outfits for $25 apiece.
For elaborate, world of disney at downtown disney.
For price, Kmart. Last year they had good prices. Don't tell her and hang them up in the room when you head out to a park. When you get back, there will be a surprise for her to find.