Greetings Timothy:
Hammons Product Company, as you mentioned, buys a lot of black walnut seed. University of Kentucky extension forestry also publishes a list of seed buyers, though none are from Illinois.
Other possible buyers of black walnut seed include state and private nurseries who produce a lot of bareroot seedlings: WI DNR State Nursery, IN DNR State Nurseries, and IL DNR State Nurseries.