Water Buffalo horn which is extremely black and is available as Demonwoolf suggests on ebay. It is used a lot by bow makers here in Britland for making bow nocks as well as other parts, also a good thing it is not heat treated to produce a flat block the horns are naturally long and very dense. Strange thing is it is a by-product of the ever increasing Mozzarella Cheese industry so its cost is quite affordable.

The only lessons in my life I truly did learn from where the ones I paid for!