Gearboxes are highly expensive and hard to find in my country. so me and my friends had to build a gearbox for our robotic arm project. Gear ratio is 1:12. It needs 6 bearings (which as known as 624zz). 3x M3 nuts and 6x M4 nuts and screws. We didnt fully tested it yet but we printed some parts and seems ok. Atleast u can get an idea to build your own gearbox. good luck.