Vex has educational inventor's robotic contests and parts for grades 3+; and 6+ and 9+, and manufactures a great assortment of parts that can build many things including gears, gear boxes, structural parts, electronics, and anything else you might need. A bit pricey yet popular. Various useful technical data can be found around their website: https://www.vexrobotics.com/
I like the Old School heavy duty Cortana Line
Authorized distributers: https://www.vexrobotics.com/how-to-order
Sample authorized distributers (for Left Coast USA):
Idesign: https://www.idesignsol.com/edr-motion
Cost of a vex 2 wire motor 393 276-1177 is $16.99
Robotmesh: https://www.robotmesh.com/
Cost of a VEX 2-Wire Motor 393 $16.99