If you can't find a gear (or don't want the hassle of looking), remove the broken gear (they usually break from an "O" shape, into a "C") -- measure the metal shaft, and select a precision drill bit the same size or very slightly smaller (one or two thousandths). Slide your gear onto the drill bit 1/2 inch from the plain end. Now, hold the spiral end of the bit in pliers (or just the tip in your fingers if you're brave), carefully heat the plain end of the bit with a micro butane torch (take care to keep the flame away from the gear!) until the gear center begins to melt, and the C closes back into an O. Stop heating immediately when the broken gap is mostly closed, and the gear is molten about halfway towards the teeth. Quickly while it is molten spin the drill bit in your fingers checking to make sure the gear is still straight, untilted and non-eccentric. If you spin it while heating you can more easily control keeping the gear straight, and are less likely to burn your fingers. You may have to heat it a couple of times to get the gear straight, taking care not to overheat it. When the gear spins straight and "non-wobbly" (that's a technical term), allow the bit to cool and remove the gear. Take a small file and roughen the metal shaft where the gear is supposed to go. The shaft must be clean and oil-free. Place a SMALL drop of superglue on the shaft, slide the gear onto the shaft into place and rotate it to distribute the superglue. Bonding is almost instant so rotate as you slide the gear on.
Gears are usually high density polyethylene (white, but the molten plastic is clear), and will not bond to superglue; but your drill bit should leave enough rough surface for the glue to grab. (If you have a round needle file, slide it in and out of your newly restored hole in the gear to add a few up and down ridges to help the glue, do NOT rotate either). The superglue will provide enough of a bond to work. I've used this approach to fix anything from alarm clocks to Takara Animated Eggs. (The eggs often break BOTH parts on the shaft, one part is the winding ratchet, the other part is the pulley that is attached to the winding string -- I heat the actual shaft twice, fixing each part one at a time.)
You now have a working gear without having to search for a replacement, and because your drill bit is larger than the original hole, the gear has much less stress than it did (which is why it broke). The procedure is easier than it sounds, just watch the gear and stop heating it immediately when the center melts and the broken ends close mostly together (a tiny gap is okay).