Try a local swap-meet. It's unlikely you will find an empty Bachmann/Mainline box for sale, but there are some people who sell non-branded boxes for storing locos and other items in. These are often made of thicker card than original boxes and have a lift-off lid. There are also people selling these - see a few adverts in 'Railway Modeller' or other magazines.
Regards,
John Webb
Regards,
John Webb