Yes, there are a number of stores that sell the dresses - all the dresses sold in the park are the same from store to store. The main street store usually has some (not always everything). Your best best in MK is the store right across from the BBB called Castle Couture - they usually have all the favorites.
DTD usually has a very large selection right outside the BBB entrance. I have also found that this is the best place for accessories and shoe selection as well.
I am going next week (2/19) and will post new pictures of the hairstyles and brochure.
When we went last October, we did a late morning DTD BBB apt, and then did some resort hopping on the monorail. It was fun. The cast members always make a big deal of the "princesses" which the girls loved. You could do something similar and end at the Grand Floridian with 1900 Park Fare, which was my favorite character interactions:
I will check on A&E merchandise at BBB when I am there. I do not believe you can order from them without doing the package.