I have purchased from Fabric .com for backings. There is a huge range of quality , some are really nice , and others seemed just a step above gauze... so loosely woven . Always look for the manufactures name , avoid those that do not have a manufacture listed , they seem to be the lower quality. South Seas Imports seems to have the better quality. I have purchased Kona in wide width for backings and been happy.
I found a Kona muslin at Jo'anns that was quite nice, tight weave, thicker fibers than regular muslin, no slubs, and was 118 inches wide. They did have it in bleached ( white) or the natural color.