Most large gear comapnies will carry metric gears with an 8mm bore to fit the output shaft of the CIM. However don't be limited to just metric gears, as a standard gear with a bore a bit smaller will also do the job. I used a 12tooth gear from Small Parts (a Martin Gear) that had a bore of .313 which is just under the ~.315 inch conversion of 8mm.
For any gears, metric included, try
www.sdp-si.com,
www.pic-design.com,
www.wmberg.com,
www.bostongear.com,
www.nordex.com, as good old
www.smallparts.com.
-Rc