For styrene try a hobby shop, Evergreen, Plastruct or possibly a large craft store like Michaels or Hobby Lobby. My local train shop has a full line of Evergreen and Plastruct. Also some of the forum sponsors here may also carry these items. However, the shipping might be more than the styrene.
I don't have any Realtrax switches and I am not familiar with them, but maybe some thin cardboard that you might have around the house would work in place of the styrene. I would think anything non-conductive like that might possibly work for you.