Greetings Toby:
Are you looking for a list of professional foresters or a list of licensed timber buyers, as the two lists are vastly different? If you are looking to sell and market your standing black walnut trees, then I would strongly encourage you to contact a professional consulting forester to assist you in the marketing of these trees to multiple, prosepctive timber buyers as this approach is by far the safest and most cost-effective for a landowner.
Current List of IL Professional Consulting Foresters:
http://web.extension.illinois.edu/forestry/pdf/tfb-nres-202.pdf
Best of luck!