There are lots of places to get a greenscreen. Many people make their own out of bright green cloth picked up from a fabric store, but I recommend holding out for a proper greenscreen, as the results are more likely to be pleasing.
First place I would recommend is www.tubetape.com
. They have nice screens, are great to work with, and you can find them at a reasonable prices. You can also check ebay, or various video/film supply houses, but its unlkley you'll find a much better deal than what Tubetape is offering.