I also prefer 108". My local store doesn't carry much so I buy it on line. Try typing "108" quilt backing. You'll be surprised how many on-line stores carry it. Keepsake Quilting carries quite a bit. I generally get mine from
www.backsidefabrics.com. She carries a good range of fabrics and often has good sales. I even buy it for smaller quilts. The rest can be used for other projects or I donate it to a group here that makes quilts for homeless children.