Where can i get gear set for reduction ?
I want to use reduction for my CF 2812 motor so that I can use a  9'' propeller to it...

The specs say you can use a 9x5 EP prop (directly, without reduction gears) as long as you use a 2s lipo (instead of a 3s lipo) and an ESC close to 20A.

Please do test slowly on the ground, for overheating of any component.