Go on line and tell people you need solid colors for backing. Be sure you specify the type and weight. I did this several years ago and I got more fabric that I have been able to use.
Also thrift stores and buy sheets or draperies that would be suitable for donated quilts.
I also make charity quilts and spend as little money as possible. I also buy crocheted afghans, bring home and wash and donate them. There are so many ways you can help people. Mine are donated to Veteran's Hospitals and nursing homes. Some time Nursing home won't take that kind of a donation. You also can send one to a soldier overseas. Some churches have Charity baskets that are sent every month to our service men and women. God Bless You!