You might also want to check out the ReflecMedia Chromaflex system. We own two of them and they make doing greenscreen work SO much easier and simpler with less heat, lights and better comfort for the talent.
I use paper for my green screen. I bought it a Samys camera in Pasadena Ca. and it comes in 10ft wide rolls. It is green chroma, works perfectly and only cost me $40. I just shot a short using it and pulling the key was a breeze.
Here is a commercial for a contest that I shot using it.
I used fabric from walmart ... we sewed three pieces together and pinned it against a wall... total it was about $30 and the greenscreen is about 14 X 12 feet... the keys are easy as well...but then again you can key just about anything with Keylight.
Depending on your size needs and how cheap you want to go, I've had good experiences with green posterboard. If you tape them with green tape, or from behind, the seams aren't noticable. On the other end of the spectrum, I have a 6'x7' collapsable green screen from Botero, and it works great! ($70 at B&H).
I bought a few 4-1/2 by 9 foot sheets for $15 each from TubeTape.com, they're working out very well with my DVX and FCP, you can see some shots of the results at http://nevercoldcall.typepad.com
Ebay... That's where i got mine and I believe it was Amvona actually that sold it to me in an auction. It works fine. There are others though that look good too.