Are you after the actual greenscreen, or are you after video clips you can use as backgrounds?
For greenscreens, I'd recommend checking out www.tubetape.com
, they have quality products and affordable prices.
For video clips, you can use any video you want as a background, essentially, so you can look at stock sites that sell clips, such as www.istockphoto.com
if you want, but most of the time you'll likely find that shooting video yourself is the best option for the background. It really depends on what type of background you are after, though, and what sort of composite you are working on.