prom formal garments with affordable price

Looking for homecoming/prom dress. Size 12-16. School district has changed rules. We can't have any short dresses(have to be at the knees), 3 finger wide straps, no low backs, no low fronts, no slits. She would prefer red, black, blue, or purple. She likes flowy dresses, no poofy and not tight. prom formal garments with affordable price