Adam: Try Matsu Momiji Nursery. Its owned by Steve Palacik and he specially grafts cork bark black pine for bonsai. You can find cork barks online by searching that term, but you'll really only find ones grafted for landscape purposes. These are typically grafted too high up on the understock and are not suitable for bonsai. Steve's are grafted low to avoid/minimize the unsightly graft union. I have a number of specimen cork bark and standard black pine that I'll be offering next spring; however, they are all 25 to 50 years old and styled. Good luck. Regards, Matsu
Steve's website is:
www.matsumomiji.com/