I have found an interesting supplier in Germany who sells horn, alternative ivory, stag horn crown pieces, hornstrips, bone and even sinews. The name of the company is Dictum GMBH. Their website is in German but also in English. They sell online too: http://www.mehr-als-werkzeug.de/category/Materialien-2526_3624.htm
They offer a wide variety of things, but you must be sure with the pricing. Some of the prices are per 100g and some per piece, so if you do not exactly ensure you can order something for a more higher price than expected. Especially for those exotic raw materials.
I ordered camelbone for my bone topping, it was the cheapest option for me. With this variety of products Dictum is a reseller of goods so searching for alternatives often helps you finding other cheaper sources.
Customer service is good and fast, at last for german customers like me.