There are LOTS of different types of builds here, however you're gonna find mainly foam builds, the other main type of build is pepakura (papercraft), but that is going to take LOTS of time & LOOOOTS of patience. Other than myself I don't know of very many spartans that have used plastic in their builds. Sean Bradley makes a REALLY nice vaccu-formed ODST set, but I know from my experience in the 501st, that vaccu-formed armor is usually pretty expenisive, however I haven't priced one of Sean's kits.
After a bad experience with a foam Halo 3 Marine build, I never wanted to work with foam again & I have a TON of expereince using a plastic material called Sintra so my original plan for my Spartan was to build it completely out of Sintra. Eventually I decided that I wanted the thickness & comfort of EVA foam, but the strength & painting surface of the Sintra, so I'm using modified foam templates to put a Sintra shell over the EVA & now I have a kit that will be the best of both worlds, feel free to check out my build (link in my signature), maybe you can find something there to help you with your build.