I would recommend you use a cotton quilting fabric, JoAnns has many nice ones and you can use a coupon, if is not on sale. I would match a color in the flannel.
While muslin is inexpensive, it is not very attractive for a quilt backing after you spent so much time on a nicely pieced top. If you are quilting it yourself and not doing freehand machine quilting, be sure to use a walking foot, flannel can stretch easily.
