I often buy wide flannel backings from Fabric.com- they have 108"-118" wide flannels that are really nice. I love Benartex double sided flannels for quilting- they are so *meaty* and wash up beautifully= I use them often for piecing & quilt tops- but I love the wide flannels for backs so I don't have to piece them- and fabric.com offers free shipping at the $35 level- I can always find that much to buy.
