If you can't find any muslin acceptable, you can use sheets. Wash them in hot water and toss in a hot dryer, that will get rid of all stiffeners and if used, any other stuff that might be lurking in the material. These will last and last and last.
I was raised to believe that the only thing acceptable for quilt backing was white, but now it's becoming not only acceptable but quite usual to find either patched backing or some color/pattern matching the front of the quilt on the backing. It's YOUR quilt, use your color sense, or use whatever you can find and feel good about.