You have many suggestions as to where to get your wide backing, but I'm just wondering how you are going to put your quilt sandwich together - tie it, hand quilt it, quilt it with your sewing machine, rent time on a longarm quilting machine or take it to a professional longarm quilter? Quilt backs that have a busy pattern will hide a multitude of sins, muslin will show every stitch and slip.
Whatever you choose, have a blast on your quilting journey!