I used cardboard (not the "slick" glossy kind) and any (again not "slick" glossy advertising inserts) newspaper that has been put through a shredder. I've been using that along with shredded junk mail (no shiny). If you shred envelopes with a plastic window, you will find the shreds of plastic in castings as they won't touch them. Using a shredder keeps the bedding fluffy.
