Stock Drive Products / Sterling Instruments – SDP-SI has a great resources section on their site breaking down a whole slew of different technical articles on power transmission. They cover clutches, bearings, couplings, pulleys, belts, and gears. They are a great resources for smaller gears and pulleys, specifically smaller than 125mm or 5 inches. Below is a screen-cap of their selection of parts.
The image below shows the range of just the gears that they offer.
If a gear is out of stock then it will be queued to manufacture.