I think is what you need:
you can get it from HH direct or if you can take the servo ( with or without aircraft) to a LHS you can probably look through their parts and hopefully find the size you need. They vary by size and tooth configuration depending on the size of the servo and manufacturer, so I would have to just visually pick what looks close through the package and check it. At least they are not expensive.
I gave up wearing contacts before I started flying, but now I have those nice, warm, humid mornings where you turn away from the sun and have both lenses on my glasses fog up; can't see the airplane with or wiith out