Contact Brent directly to confirm he's sold out, and ask if he has anything larger. J. beech are slow and tricky, so you'd want one grown by a nursery that did the right work from the start...such as Evergreen or Telperion Farms.
http://www.telfarms.com/trees/display
Don Blackond may have some, send him a PM.
And finally, Wee Tree (showing as sold out, but it's hopeful that they're growing them). You'd have a LOT of correction work to do buying one of these field grown trees. I'd be very careful going this route with a J. Beech:
http://www.weetree.com/index.php?l=search_list&s[title]=Y&s[short_desc]=Y&s[full_desc]=Y&s[sku]=Y&s[match]=all&s[cid]=0&s[search]=Beech