I have found that digging them up from the yard is a great way to find them. I dug up 3 individual trees and a 3 truck raft late last Feb. They had a great growing season and are still doing really well. I am also in Western North Carolina. Carpinus Caroliniana grow all over the place out here.
Are you going to be at the winter silhouette show in December in Kannapolis? There is a vendor there who usually has quite a few collected American hornbeams. Have you also checked in with the blue ridge bonsai club. Collin from the club ground grows them as well and may have some, or will be digging some for spring.
I am trying my hardest to go. I went to the last blue ridge bonsai show at the arboretum, I was just talking to someone about them and I am trying to figure out who it was, perhaps it was him.