Is this for a high load application? Plastic gears can work out well for the first stage of a gearbox for medium power motors like the BAG and 550. This one may survive:
link
We have had success with PLA 3d printed gears in the past as well, and are planning to have a lot of them on the robot this year.
The RC world has a lot of gears in that size range, but they tend to have specialized bores that might be hard to modify. (
for example)