Aged newspaper (so the ink is permanent), old phone books, dryer sheets, cheap sew-in interfacing, old thin sheets or thin fabrics that you don't want to use in the quilt top, parts of old shirts, aprons and other worn clothing, white tissue paper, etc. Lots of choices. I really like old phone book pages for smaller blocks. If you have a newspaper office they will often sell "end rolls" of newsprint paper cheaply and it lasts forever and is large enough for bigger projects.