Do a net search for Modest Formalwear or Modest Prom. There are various standards of modesty out there, though, so check carefully. For instance, Muslim standards require one to cover everything but the face, hands, and feet. Jewish standards permit one to expose the lower arm below the elbow, the lower leg below the knee, the ears, and the neck down to the collar bone; but unlike Islam, which only requires that one be covered, strict Judaism requires a woman to conceal the area where her legs meet together (as with a skirt or dress). Mormon standards allow a girl to show the knee below a skirt or shorts, wear trousers, and even wear short sleeves and a lower neck - as long as the shoulder and the breasts are covered. So learn what your community's standards are, decide how well you want to follow those standards, and then go hunting.
smile If your family is willing and able to pay a fair-to-steep price, I suggest
http://zorifashion.com/gowns.php -- Most styles suit the 30-and-older crowd, but some would be suitable for a younger person.