I've had an interesting ongoing email conversation with a person in Mexico who bought my plans and was very interested in building a solar parabolic heating system for himself. He had been having trouble getting the specific gear motor that I recommend in the plans.
In any DIY (Do It Yourself) project sometimes it is necessary to improvise.
It occurred to me to suggest to him that he look for a discarded cordless screwdriver or drill. Often people throw these away when the batteries no longer hold a charge. Both of these products contain a planetary drive gear reducer and a DC motor, just what we need to build the drive mechanism for a tracking solar parabolic collector. In both of the tools I look at here, the gear drive has metal gears on the output side so they should be able to handle lots of torque.
It would also be necessary to compensate for the lower operating voltage of this motor. That could be as simple as a resistor in series with the motor.
The whole reason for the gear motor is of course, the very slow speed of the sun across the sky. If the solar tracker drive operates too fast, the array will overshoot the position of the sun, go back, overshoot again, go back etc, etc. So the drive would be unstable and it would in effect, oscillate which is not desirable since it causes unnecessary wear to the whole mechanism and uses more power than it needs to.
So if you can't find a suitable gear motor, look around at your old cordless tools, or visit the local re-cycling center.
My friend in Mexico liked my suggestion and is now looking for a suitable tool to re-cycle for his solar project.