Where can i get a nice dress from??

Im 13 years old and my close cousin is having a big bat-mitzvah party and i need to get a really nice dress, but from where i dont know! It needs to be something really special. Im not trying to brag or anything but parties like this one in my community are pretty extravagant and somewhat equivalent to a Hollywood wedding! There will be over 200 guests and since i live in a pretty snobby town people want to look there best because if they dont they know people will talk. There will be people in designer gowns and all that stuff (im invited to a wedding the night b4 and im wearing a ralph lauren gown). I dont want a gown for this i would rather something shorter and young. The problem is i really dont want to spend too much money and i really want to look good! I would like to spend no more then $200. Please dont call me some snobby spoiled bratt because im really not! please give me some good ideas!