Sorry dude sounds like you need a new mixer, unless you want to try that zoo they call eBay.
Try looking on ebay….If not there you might be out of luck.
1-800-488-1222 or
most likely if the cement mixer is old they dont make it anymore ,my advice is to go to a metal worker and have him make it for you .
Try grainger.com. They may have gears, shafts, and bushings. Machine shops can make shafts and bushings. Welding shops to weld a new gear to the shaft. Good luck!