Sears Craftsman Liftmaster garage door opener repair gear kit 041C4220A / 41C4220A online suppliers:
(use the blue box ← → arrows to scroll through ads)

The Liftmaster 41A2817 gear kit is just the large gear, and while cheaper, it is pretty hard to get it off the shaft.  But the same suppliers should sell the 041C4220A, that is what I used, (gears are already mounted on the shaft and ready to drop in as described on this page).  Try a search for 41C4220A on the sites in the ads (the sellers in the Google Ad below should be pre-screened to have these parts and be "Google" approved vendors).   Last I checked it was about $29. And the Sears link gets you to their parts search and you can get it there too.  Clicking these ads supports my efforts to put up this help page.