I use Thousands of Bolts for backing. Most quilt shops and JoAnn's does not carry the selection that you can get at Thousands of Bolts. Their pricing is very good. Do use the search ...since they really do have thousands of bolts. It will get very tiring to search all the fabrics.
I prefer a single piece , when I am done doing the top ... I am done and do not want the hassle of the long lenght, particlulary if my sewing room is not tidy ...as is the case often ...at the end of a quilt top.
Unless you are using orphan fabric ... there is not much to be saved if you shop wisely for a single piece backing.