When all else fails, use a bed sheet!
I make charity quilts and buy used sheets at Thrift stores. It saves a lot of money and does not need piecing. Used sheets are most often soft from repeated washing and work well to give a nice soft backing to the quilt.
Those who are making "heirloom" quilts don't like this idea, but I find they extend my budget and saves time in piecing.
I also have used the 108 inch backing on big quilts when a good sheet in the appropriate color cannot be found.