I buy them regularly at Hancocks with my 40% off coupon and watch for sales and special discounts. I prefer a one piece backing. I have also purchased and JoAnn's with less success, they seem to bleed color. Also watch your backing that it is not thin. If you hold it up and can see through it you may have your quilting punch through the batting and show on the back side.
I mostly use the 108 in. backing fabric and I love it...I get all my backing fabric in NY from an amish store...Her fabrics are cheep and of great quality....
I use wide unbleached muslin. I like the old fashioned look to it.
Extra wide backings
i use these whenever I can, one reason is the cost. Most of the extra wide backings work out to be about half of what it would cost if you bought fabric at 42" wide from your local quilt shop. For a king at say $18.99 a yard for extra wide would be about $66 figuring that you would use 3.5 yards. Given that same scenario you buy 42" wide at $11.99 a yard times 10 yards you would pay $119.90. The other is the ease there are no seams which makes it easier for the Longarmer or domestic machine to quilt.
The quality I have found on most of the ones that you get at the local quilt store to be a good quality. Sometimes the ones that Joann's sells sometimes you have to watch the thread count. However that being said I have also found some really nices ones there too. Its like everything else you have to look at what you buy and the cost.