It's a fabric, called Molton. It comes in black, white, and chroma key green and blue. I get it from a place nearby (2 minutes away) in Oakleigh, Theatrical Supplies of Australia: http://www.theatricalsupplies.com.au/Digital-chromakey.html
It is a brushed cotton, a bit fuzzy and felt-like, and is cut from a 3 metre wide roll. I bought 2 metres, using the 3 metres as my width, since the plywood panels I staple my backcloths to are only 2.4 metres height. I see there is also a polyester Digital Green cloth available there, which I haven't tried. I had a small sample of the chroma-key blue, and it isn't as bright and clean a colour as the green, you can see that on the website.
The other part of getting an even screen is lighting it fairly evenly, with no hot spots, big drop-off in brightness, or shadows from the set. I have 1 metre minimum gap between the back of the set and the greenscreen. I mostly use 2 150 watt halogen floods from Bunnings, one each side, to light the cloth. When I use the cloth horizontally to cover a board to stand the puppet on, it comes up much brighter because of how the light hits it, so for the last set of shots for Dark Earth I painted the board a darker green until I found a shade that came up about the same as the backcloth on camera.