Baelor Miniatures have added a Stone Wall terrain piece to their Armorcast online store.
From their announcement:
For use in the current edition of the world’s most popular RPG, or any game that requires several 1 inch square stone wall sections, this pack includes 8 segments. Not pictures is the other side of the wall, which is detailed differently for a more realistic look when several sections are used together.
Available at the Baelor Armorcast Store.