Dan @ Keshaw Designs and SDP-Si.com are the only places I know of that sell them. I had a 21 or 22 tooth brass pinion that was 5mm bore but the grub screw rounded out and I had to cut the gear off. If I still have the gear you can have it and use some loctite 604 to stick it to your shaft.
All I ever wanted was an honest weeks pay for an honest days work.