I use flannel often that I buy at Joannes when they have their fall sales, and large selection, I cannot afford to use LQS flannel for backing. I realize the quality of a LQS flannel is better than Joannes, but unfortunately it is out of my price range for backing. I have used minky and fleece too. I also have bought flannel sheet sets that are good quality at my local thrift shop and yard sales and have used them as backing.
I typically use quilting fabric for backing when I want cotton.
I have read about using a flat sheet for backing too, which is made for several washes, would be durable, and less expensive than quilting fabric.

Like Tartan posted, it depends on the size of the quilt. If it is a large quilt...queen or more, I will purchase wide backing. I have a modest stash but when I do scrappy quilts, I will always try to put together a back with pieces from my stash. There is a LQS near me that has a sale room where I can easily find fabric for backings...

I buy JoAnns Legacy 108" muslin on sale, usually try to get a full bolt, and use this for all of my quilt backs. Washes well, easy to quilt, and to me, looks good when done.

I am always happy if I can find a nice cotton flat sheet at my local thrift stores. I go in those stores, head straight for the linen section and will buy up whatever sheets I like. If I don't use them for a quilt back, I use them to make pants for a local shelter. I look for 100% cotton, but am not opposed to a nice blend - but despise working with sateen, so avoid that like the plague.

Muslin a good quality muslin is not too dear and provides a wonderful feel, I love Joann’s Legacy Premium Muslin it’s very soft and quilts up beautifully, comes in a wide variety of widths in addition to being a great value when purchased with a coupon! I buy it by the bolt and use it very often for the back of quilts.

I agree with Dunster! Wide backs are better than they used to be! I also buy great fabrics on the sale shelf and use it on backs and I love using my leftover fabrics from the front.

I have used new cotton sheets for backs of very large quilt. My preference is wide quilt backing fabrics. If I am planning a very large quilt I will by 120" just to be sure I have what I want. As time has gone on with my quilting I have been getting smaller and smaller. The larger quilts are getting harder to handle for me. Now If I am making a larger quilt the 108" backing fabrics are what I order. I find them to be perfect for the money spent.

I don't go skimpy for my quilt backs. Think of all the work we put into our creations! I just won't take that chance.

I usually buy the wide backing fabric for my quilts but if I have a large bit of fabric I have used in the top then I just piece the backing with most of that fabric. In fact, sometimes I have extra blocks I have made from the front and I piece those in the backing too. I think it is worth the money to buy good quality fabrics and they last a lot longer too. Good luck to you with your new fabric and hope it works out for you.

I use a coordinating print from the fabrics used in the quilt. The edges of the fabrics are serged and then washed to allow the first shrinkage to happen. Print designs are matched across seams. Seams are stay-stitched first and then I use a 1/2 inch seam. Press the seam to one side and then top-stitch close to seam line.

The extra effort and details are worth it to me for my quilts.