You want canvas, because of the interlocking weave. You can cut canvas and the edge will not unravel. Hobby Lobby has cotton canvas, and most fabric stores carry Polyester Canvas, sailcloth for about $10 per yard 36X45 inch piece.

Pellon fabric interface, stiffener is also a great strop substrate. It is a paper/fabric product that has no woven structure and holds paste very well with no integral grit like leather or balsa, works great with nano grits like CBN. It is available at all fabric stores, Wal-Mart & Hobby Lobby.

Most fabric stores have 50 percent sales all the time, check their websites. Denim works, but frays easily and must be sewed or glued, creating a seam.