An On/Off switch that can be controlled by an Arduino
There are several options, among:


Use a transistor; this is the cheapest probably, but involves checking what transistor you need.
Use a relay, you can build your own circuit 'around' it.
There are also a complete relay module (they cost 50 cents from AliExpress, but for a motor I would buy something decent), and you can find easily an example how to connect it.


Note that you need an external power supply (unless the motor is really small).