Home Depot - Foil sound barrier styrofoam panels............................... styrofoam insulation with sound barrier foil on them. They are 4' wide by 8' tall, weigh about 3 pounds each, and are only $10.00 a panel.
I used carved styrofoam insulation board as is pretty common now. The only thing I think make mine stand out from some that I've seen on other sites is that I didn't want them to look too new. I wanted the look of old, cracked and pitted. Like they had been forgotten in a run-down cemetery for ages. The painting was done in stages - a medium base coat followed by two coats, one light and one dark, applied with a sea sponge.
foam insulation, some cloth, and a styrafoam head make a nice creepy tombstone. just remember to not use spray paint, it will eat the styrafoam. mix some fine sand into paint before you brush it on or scatter some over wet paint to get a stony look