Black walnut seedlings can be purchased from the IDNR state nursery in Mason County or private nurseries such as Forrest Keeling, Hensler Nursery, etc.
Based on my 20 years of professional forestry experience, I honestly believe most people are better off just buying "woods run" black walnut seedlings and planting them at 10 x 10 or 8 x 10 initial spacing. Management of your black walnut plantation, matched with proper soils and site, will likely have a far greater return on investment than purchasing "select" or grafted walnut trees -- but, that is just my opinion!
The most obvious factor that most people overlook is "site" and "soils." I strongly encourage anyone interested in establishing hardwood plantations to utilize the USDA's Web Soil Survey:
Best of luck!