Champ Body File Sand Paper Sheets - 2 3/4" x 17 1/2" This type of paper is the cats meow for flattening things. (bones, burls, even hones). The paper comes in various grits with an adhesive back and is long. I start the 36 grit and stick to a table top. After using this you can put full or half sheets of a finer grades of paper on the table and rub the bone on them to flatten them.
I have to second what PDobson said about backing up the paper. That is a great piece of advice.
The local auto body paint/supply store sales it by the individual sheet.
Charlie