You also might check ebay, I've got some awfully good buys and wonderful fabric.

check marshallsdrygoods.com they have lots of wide backs under that price. Do an internet search with the term wide back fabrics and you will come up with some others.

I agree completely. I ordered one from Fabric .com ... quality was not nearly as good as Thousands of Bolts

I like to use flannel but have started piecing my backs.

Usually from clearance fabric. My local quilt shop sells flat folds for $3.98 a yard. This is for high quality fabric such as Moda, Timeless Treasures etc. They take it off the bolt when it gets past season, or there isn't that much left, and move it to the flat fold table. I look there each time I go with backing fabric in mind. Rarely is there enough to piece a larger quilt, but as someone else mentioned, you can piece your backings and have them look quite nice, and really, more interesting than just a huge backing of one color. For lap sizsed quilts, many times there is enough. Our LQS also discounts fabric on bolts, and I check it as well, considering what might look best for backing. I used to use muslin, but every stitch shows! and it shows dirt easily. Sheets have such a tight threadcount that they are difficult to quilt through. I would suggest keeping an eye out for backing fabric when you are shopping, keeping in mind the project you are making. Most of us aren't too inclined to pay a fortune just for backing.

In the UK, Ikea do 150cm (about 60in) wide cotton fabric, until now mostly plains, but they've just brought out a very pretty range of polka dots and florals. Don't know if they sell them in the US as well.

I piece mine most of the time now. Sometimes the back has some of the fabrics from the front, sometimes not. I usually start with a large rectangle of fabric, the width of the fabric, like 60x40, then extend all sides with 10" strips and squares (cut to 10.5 inches). I sit down with a tablet and graph it out. Easy to do in even increments of 10". I start with a rectangle because my quilts are rectangular, not square.

Joyce

There are a lot of sales on fabric now on online fabric shops and also at JoAnn's and Hancock.

I buy most of mine from Ebay as they range from AU$3.99 a metre (sorry, we dont use yards here in Oz).

I shop online a lot and if I find a fabric for $3 a yard and it is a good choice for a backing I buy enough to use for backings.
If I want a large size fabric backing and need it now, sometimes will use a coupon at Joanns for wide muslin.