Luca, I am sure you don't need fossilized ivory! You can buy mammoth ivory from knifemaking supply shops, but I have found that in most case it is too brittle. If staghorn is too soft, cattle bones are quite hard if they are degreased and dried properly. In some difficult cases, soft metal tools (copper and brass) can help on iron TSUBA, but they may leave a metallic residue which has to be removed afterwards. Best of luck with your project! Let us know about your results in a few years!