I can't recommend Chromaflex aka Holoset highly enough. It looks a little expensive upfront but what it will save you in terms of not needing to light your screens perfectly and not spending decades saving bad composites in post will pay for itself on your first gig. Also your clients will love that you spend only 10 minutes setting up a keying shot and not two hours.
http://store.virtualsetworks.com/detail.aspx?ID=1102
Otherwise if you simply google for greenscreen you'll find a host of standard options.
Noah
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