Hi Oystein,
I am not aware of any gearboxes that will fit our motors without modification of customized adapters. It is unlikely that any are available- if you wish to gear down an M100 or N200, you will likely need to modify an existing gearbox or make your own! Vex robotics kits parts are often used for such things, though they may be a bit large. A planetary gearbox or worm gear drive will be more compact.
-Adam