I used to use those plastic backgrounds, but they always looked like crap. On my last 3 tanks, I got a can of cheap spray paint from walmart (usually a darker blue color) and painted the back of the tank. Always comes out great, no plastic to fight with and if I don't like it after a while I can use a razor to remove it and repaint, etc.
If you still have your heart set on plastic ones, most petco and petsart stores carry different ones that you can buy by the foot. Also pretty much any LFS usually carries them too.