They come with several patterns and instructions on how to make them. There are lots of magazines like Burda Style, venom costumes Simply Sewing or Patrones in Spain that specialise on patterns. Get patterns from magazines. Etsy is a good place to get a few resources at reasonable prices. Unfortunately, they may not have the pattern you are looking for and you may still need to modify it. The job of a costume technician is to construct and pattern the costumes for the play or performance.