i know you are looking for a SINGLE round ball but....
Dixie slugs makes their 'tri-ball' round, which is a 12 gauge load, that fires 3, 20 gauge lead balls. Accuracy is claimed to be very good for what it is in a tight choke.
If you swing on down to www.shotgunworld.com
there is a dixie slug forum where you can talk to the owner/creater of the company.