Chris,
The product used to be called "Celotex", but may be marketed under a different name. They make two types...asphalt impregnated and not with ashphalt. Go with the non-asphalt stuff. It costs about $8.50 per sheet now, but is tough to find. Stack several sheets and cut it with a circular saw. Make the pieces 18" wide by 48" long. You will get 6 pieces from each sheet and 16 sheets will provide a 4' X 4' target. After places get shot up, you can rotate them around to extend the target's life.
Another cheaper alternative that works really well is sheets of corrugated cardboard stacked the same way.
You can tighten/loosen either target to get the penetration that works best for you. Both types of target should be covered from the weather.
Hope that helps.
Joe