Wizarding World on Twitter: "For the Yule Ball, the outfits needed to mirror the characters personalities. Jeny Temime, the costume designer, said she also wanted it to be a "fashion statement" -
Making Hermione's Yule Ball dress: the bodice – Happily Ever Taffeta
Harry Potter Couture: Winter (Yule) Ball - The Observer
How did all the girls get their dresses for the Yule Ball? Was there like a shop or something in Hogsmeade? - Quora