Hi Dawn,
I think it is usual for Bullmastiffs to be shown on a thick choke chain and leather lead. You can get these online or at large shows. If your dog is still a pup you might want to start off with something softer like a thick leather slip lead. Have you attended any shows? If not, try to get to at least one with a reasonable entry of bullies and take a look at what everyone else is using.
Jen