Low speed cheap motor
You haven't specified motor power, nor range of voltage for you motor. I do assume you want to power the motor from battery, so recommended option here can be a 12V DC motor with gearbox. Try to find them on any popular online sites by searching 'DC motor gearbox low rpm'. By quick view there are plenty of options for less than £10. There is a selection of metal or plastic gearbox with compact build.
It will cost more if you are about more powerful motor or higher voltage motor.