Small cuttle bones - 3" - 4" approx.
There are two readily available forms of calcium - Calcium Carbonate which is most effective and is used in many of the vitamin supplements, and Calcium Phosphate which is found in cuttlebone. The destructive act of chewing a cuttlebone is said to be quite therapeutic.
available in single pieces or in an economical 500g bag (in excess of 50 pieces)