Try small boutique and wedding shops. Places like this typically have only one of each dress so you are guranteed to stand out. They aren't all that much different price wise either then a dress found at a shopping mall. I got a beautiful one-of-a-kind dress for my senior prom for $250. This included the alterations.
If you go to a wedding shop there will be a vast assortment of bridesmaid dresses that can easily pass as prom dresses. They can measure you and order a dress that is just right for you. Ordering dresses can take time, however, so shop early.