Where to buy planetary gearboxes?



I have indeed spotted the 42BYG with gears option and i am not interested.
I want to buy and integrate a planetary gearbox with my own motors exactly like i do with gears and time pullies/belts. However, I find myself unable to find planetary gearboxes. I could make it buy buying compatible gears but (1) i dont know where to buy the arm that connect the planetary gears together to the output shaft and (2) it sucks i would prefer to buy a single ready to use gear box package

=> where do you guys buy them ?




Robot shop ?