A good local company we used for our locust fence posts is Top Notch Tree Service. They also do tree work - he's an arborist - and they harvest the locust from their own locust grove. He has very decent prices, $4 per split eight foot post and $20 delivery.
You can contact Rob Neff at phone 607.898.9898 cell 607.423.9400 or email at TopNotchTrees84@yahoo.com