Hi, I wouldn't buy anything out of styrofoam, it is too weak. The only styrofoam I have ever seen used was under a synthetic stucco spray application for whole-house exteriors, not detail work.
I have seen foam filled door trim and misc. mouldings in home supply lumber yards. This stuff is pretty cheap and is not for high-end use and it would probably break down outside.
What you need is high density polyurethane mouldings etc.. The best source I have seen for stuff like this is http://www.outwater.com They actually make most of the stuff they sell. This is as high up on the food chain that you can buy as a home owner or builder. It is rare that you can purchase like this without getting fleeced by middle men. I have bought all kinds of stuff from them over the years and have never had a problem. You should get their actual catalogues, in addition to what you see on the web because they have a lot more stuff on paper and it is a good reference for future stuff you might need.