black walnuts
Black walnuts and many other native nut and fruit trees and shrubs can be purchased through Oikos (pronounced ekos) Tree Crops. Their address is :
Oikos Tree Crops
PO Box 19245
Kalamazoo, MI 49019-0425
Ph. 616-624-6233
Fax 616-624-4019
The owner is Ken, and is very helpful, They have a real nice little catalogue.
I've done quite a bit of research on purchasing tree liners and saplings.
Best of luck.