Zoo Med offers this product for turtles...
Turtle Bone™
Turtle Bone™ is a natural floating source of calcium for all species of aquatic and terrestrial turtles and tortoises. Turtle Bone™ provides habitat enrichment and supplementary calcium on an “as needed” basis, and the rough edible surface helps prevent overgrown beaks as well as curb unwanted chewing behavior.
My turtles seem to really enjoy it, I just let it float in the water for them and they eat it at their own pace