Industrial: Worm Drive Motors
These worm drive geared motors are a good medium heavy duty unit. They are powerful and have different gearbox and motor options. They come in 12V and 24V DC for your voltage options and the motors can come with 2 speeds (1600 RPM and 3200 RPM). The gearboxes are available in 25:1, 50:1, 60:1 and 75:1, giving you good options of both power and speed. Unlike cheaper copies of this motor, these units have replaceable brushes and are an industrial rated unit which can be run continuously.
They can be reversed, and controlled with a speed control unit. These are also available in bulk.