I love the wide backs from SewBatik.com . They are a good quality, no seams!, and when you figure out how much it would cost to buy the fabric it's real close in cost. These are not available in quilt shops, only through Long Arm Associates (I am one) of directly from their site.
Another good wide back is the Fat Back from Christian Lane Quilting. I haven't used them yet, personally, but I've heard great reports back from those who have.
http://www.christianlanequilters.com/widebackings.html