My favorite white is Kona Snow, and I always buy it in a wide backing for every area of my quilt since it is cheaper by buying it that way. I also appreciate not having to piece strips as often since it is so long. I've never found it to be of a different quality from the 42" yardage. I buy it on Amazon.
My favorite place to buy other wide backing is Memory Lane Quilting. I like their price, quality, variety of patterns:
www.memorylanequilting.com
(330)468-2831