North American hardwoods is a very depressed market. It isn't what it used to be. When I was in forestry school in the seventies there were stories about tree theives that stole trees from front lawns. Its not like that anymore but a good black walnut tree still gets good money. If you find a specialty hardwood wholesaler then they can tell you which saw mill might be interested. I do not have any contacts in the forestry business but if you are desperate I can try for you. A tree owner can pay a saw mill to cut the tree up and then the tree owner can dry it. If its really big then it may be worth it. Problem is that a core sample may show a lot of heart rot common to older hardwoods.