The smallest I have ever been able to find is 14 tooth sprockets, from Servocity, and you will need a hub to match the 5mm shaft you want to mount it to.
http://servocity.com/html/hub_mount_...ts___250_.html
Or if you bore out a sprocket meant for a 3/16 shaft, you could get a 12 tooth, but you are on your own for mounting it, and it would probably have to be pinned to the shaft.
http://servocity.com/html/plain_bore...s___1227_.html
Here are the hubs I used for my 14 tooth sprocket.
http://servocity.com/html/set_screw_hubs.html