As she's discovered, Disney Store only goes up to size 10. My 13yo was bummed when this happened because she still wanted to dress as a princess.
She'll have the most options by searching etsy. As far as Disney quality goes, they'll be much better quality for the most part (I don't think very highly of Disney Store quality).
I second Etsy for finding better than Disney Quality dresses. Be prepared to pay higher than Disney prices for some of them, but, they can make them to fit any size.