Thank you for the question Kelsey.
Transportation of high quality veneer logs is usually not an issue when selling black walnut. In fact, most good veneer buyers absorb the freight costs in order to procure your quality logs.
Since I can't recommend one company over another, this is exactly what I would do in your situation:
(1) Contact a local professional forester who is adept in marketing timber. Ask if he/she would provide you with some recommendations of local buyers/sawmills who specialize in moving high-quality logs
(2) Contact your local state forester and ask them the exact same thing
(3) Contact veneer mills and sawmills directly (Missouri, Iowa, Indiana, and Illinois have sawmill directories). Larger companies have multiple buyers and/or have a procurement forester on staff. Timber buyers will travel much farther than you think if you have quality logs. Send them photos if they question your visual appraisal.
Word of caution...since the logs are already cut, don't be too picky in your marketing strategy. Obviously try to maximize your profits. However, next time market your logs/timber first, and THEN cut the trees after you have agreed to terms with a buyer (best to have a written agreement). Cutting first and marketing second is not recommended. I would hate to see you get stuck holding 35 MBF of black walnut and only get $0.50/foot.
Depending on the actual quality of your logs, I've been hearing black walnut veneer prices in the range of $3.00-8.00/foot. Again, a lot of different variables come in to play when you're talking log prices. Remember to market the logs outside your area as well, as there are buyers all over the Midwest and east coast who need quality logs!
Good luck and start marketing your logs, Jay