The above picture is of a couple of tagua nuts. One in its raw state and the other I have carved the surface to reveal the vegetable ivory characteristics of these nuts.
For those of you that have difficulty getting your hands on bone material to practice on these nuts are a good substitute. You can either remove the outside bark with a belt sander and scrimshaw right on the outside surface, or you can slice them in poker chip size pieces and scrim on the inside surface after you have polished it smooth.
You can pick them up from various wood suppliers, but I generally get mine from Lee Valley when I am picking up other tool supplies.