I once ended up with a lot of princess dresses when I was picking up a lot of dress up clothes from a Freecycler, you might try there. If they don't have to be Disney princess brand, I have a small collection of flower girl and communion dresses that came from thrift stores over the years (my mother's side of the family are veteran thrifters and auction goers, they send all kinds of things my way, including Catholic school uniforms for my children to dress up like Hogwarts' students, etc.).