Demi Lovato?? Where can I find...?

My little sister is in 8th grade and there is a banquet coming up pretty soon. She has to get a dress and she loves Demi lovato and I was wondering where she could get a dress like Demi had in the "La La Land" music video where she says "who said I cant wear my converse with my dress". You know, the black one. I want to get it for her, but I don't know where I can get one. Someone please help! Its for my sister!