I have always made my own from styrofoam board used in building houses ( the pink or blue stuff, not the white beaded stuff). I stack enough layers to get 8" thick (how many layers depends on the thickness of the material) then duct tape it around the edges, pin on a target and have at it. It effectively stops both field points and broadheads from both my old Barnett Demon and my buddy's Horton (model unknown, but 150lb draw, same as my Demon). As a previous poster mentioned though, it can be difficult to pull your bolts back out of these, even with field points. It's worse yet with broadheads.
Jim