You may have luck find a gear here: http://www.probelay.com/
Also, some fleaBay auctions such as here or here.
Also, you can always have someone make the gear such as: http://www.performance-gear.com/ ...but i dont know if they have miniumums.
Last but not least, contact the manu of the screen and see if they can sell u the gear. Is your warranty out on the screen?
btw...make sure you measure the gear, count the teeth, and all that before getting one that "looks" like it will work. Everything needs to be almost exact.