Lilac bridesmaid dress...HELP!!!

Does anyone know a good online store that has really good quality clothing? My cousins wedding is in 2 weeks and I can't find a dress for the life of me. I want something that's long and somewhat glamorous but not too expensive. I'm in size 12. I also need the same dress for my other cousin who is also a bridesmaid in the size 2. Please post website recommendations. I need fast shipping ASAP