Your best bet its to make your own. Go to a good thrift store...GoodWill or Salvation army. By a cheap cotton dress. Distress it really well then add bits and pieces of fabric to it with hot glue. Die some cheese cloth black and brown and add it as a layer over the dress as well. Use watered down acrylic craft paints to add stains. You will be amazed what you can do in a couple of hours. I think one of the major keys to such a costume is layers of different fabrics. For some reason layers are scary.