I got my background at petsmart. It was 2-sided, with black on one side and that ocean blue color on the other. I wanted to use the black.
The oil trick works great, but takes a little bit of patience. If your tank is empty, lay it on the front so that you can work flat on the backside where you want the background. Cut the background to size. Get some vegatable oil, and make a thin layer all accross the back of the tank. Now, set just the top of your background where it goes. The oil will allow it to slide around so you can get it just right. Using a straight edge, such as a putty knife or credit card or similiar, slowly start scraping the background onto the tank. Use the edge to work out all of the bubbles down towards the bottom and sides. Keep some paper towels handy to wipe up the excess oil that you will scrape out. In the end, you have no airbubbles left, and from the front it looks like the tank was made that way.