I usually go to
www.backsidefabrics.com. This site deals only with wide fabrics, usually 108" or wider. She has a good range of fabrics at reasonable prices, plus often has good sales. I buy these even when the quilt I'm backing is 60". I take what's left over and may use it for a pillowcase or generally I put these large pieces aside and later donate them to a group that makes quilts for homeless children.