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.