I've gotten a blue & green one from one of the ebay sellers before and works fine. Essentially it's a giant heavy cotton sheet (10x12 green & 10x24 blue), but it keyed fine with my old GS150. Like any good keying set up....lighting + subject space is more important than the screen material IMO.
In my experience with the two I have, you'll want to have a hand-steamer handy. If stored improperly, then it'll be wrinkly....like this (my son's Jawa work in progress LOL):
....disclaimer....was in my apartment then and 10x12' is draped over the front door after being pulled out of a box it was packed up in pre-move. Using After Effects+Keylight, this keyed just fine, even with the wrinkles. Had it of been steamed and stretched to size, it'd of been a REALLY clean key.
By far the cheapest and most effective option is going to the hardware and asking for the most neutral green they have and painting a wall...or even a room with it.
It'll work WAY better then a fabric green that'll wrinkle, and thus cause shadows.
Otherwise...with fabric, just buy whatever, be as careful as possible and iron it. then try as best as possible to stretch it.