I've got a pretty big head so I feel your pain, but I'm at least still within the range where Bell works. The biggest helmet I've ever seen was an old Wal-Mart one -- it was the only size they had, and it was a freaking salad bowl. I don't know if their helmets are still that huge, but it might be an option. Otherwise, I found this website that offers options -- but it's from 2006 or so:
http://www.helmets.org/bighead.htm
Good luck, Charlie Brown.