You could use an existing anvil for the mold but it would be hard to get the anvil out of the sand. Styrofoam patterns are usually a one time use thing. I could get several made if I had a good pattern.
Sorry I am in Alabama. Anvil will be out of steel. I have several anvils that were cast out of different alloys. Think they pour them with extra metal out of the ladle.
Any size would do. Bigger the better.