Paul,
SCH Bearings of Greatbridge Business Park, Budds Lane, Romsey, Hants. SO51 0HA Tel 01794 830377 have always been able to help when I've needed sprockets. As mentioned in my previous reply, if you ask for a platewheel you will get a plain disc steel sprocket - if you ask for a sprocket, it will come with a boss that will then have to be machined off.
Good luck with the project,
Regards,
Eddie.