Planetary motors are just gearmotors with different types of gears. So it all depends on what torque and output rpm you need. Also since the motors will be fixed, standard gearmotors might be a better option, since planetary gear motors are mostly used for when the motor itself might rotate and you need the output shaft to stay cocentric with its position.
So all you need to do is determine how much weigth you need to push and your wheels’ diameters. After that you’ll be able to know what’s the minimum torque you need. The wheel diameter is to calculate the leverage distance you have, since torque is usually a measure of mass/lenght. For exemple: kg/cm. So the larger the wheels, the more torque you’ll need since the lever “arm” will be longer.
After you’ve determined the motor’s specs you need, search online stores like ebay or if you have a local hobby shop or one that ships to your location it’s worth trying. You might also find something on large manufacturer websites such as Digikey.com. Then just search for “gearmotor” and find the ones that correspond to what you need.