Being honest- We will have our first experience with the BBB next week with our 4 year old(last time we went we only had boys). In getting ready and making reservations, I read everything I could find.
From my research, they have most of the princess dress and accessories BUT it is limited to what is on hand - like any other store. They might have the Anna and Elsa dresses in stock but then again they might not or they might only have larger or smaller sizes.

Same goes with the rest of the princesses.
Everything I read and the advice I took was to buy whatever princess dress your daughter(s) want before you go, especially if there is ONLY one princess she wants to be and would be heartbroken not to get that dress.
I ended up ordering a costume from the
Disney Store (from what I read it was the same ones being sold in the parks but I could be wrong). It was actually on sale so I saved some money but more importantly I know she will have the princess dress that she wanted that day.
Hope that helps!