Erik Renstrom has a Facebook Group (Tree Daddy Burls, Blanks and Fordite) and sells stabilized mammoth tooth. I have purchased a LOT of wood from him and he is a really good guy. I have purchased some mammoth tooth from him but have not yet tried turning it. He has posted several of his pens made using it. His materials are not cheap, but they are top shelf.
He is an arborist by day. He got into the wood business because he struggled to find the high quality high figure premium woods he wanted for his pens. He is licensed to import.
www.facebook.com
It is a private group so you will have to request to join. He had to do this to keep the riff raff out.