I buy my polystyrol in a special model shop close to a college for architect students. 1m x 0.5m is about 6$.
Sorry Jaiden, PVC is Polyvinyl chloride not Polystyrol or Polystyrene (EPS, XPS, PS-E)
See more here http://en.wikipedia.org/wiki/Polystyrene

This really sounds very important, thank you! You did sound like Yoda 