Some people need to be able to use their images for commercial purposes, and therefore need to have copyright approval to any images they use. That's why they buy rather than borrowing any old thing off the internet without getting permission.
I know TubeTape also sells a variety of backgrounds and overlays, and www.creativesparx.com
also has some nice looking stuff. Otherwise, any photography supplier is likely to have access to a variety of digital backgrounds, especially the larger ones like www.bhphotovideo.com
Of course, don't overlook the obvious fact that you can use your own images as backgrounds as well. When you have your camera in hand and see a spot that would make a nice background, take the shot, and start a collection of your own background images for future use.