As background for furthering bb-8 and other hobby robot projects, let's start with a basic discussion of the types of DC gear motors you're likely to choose from.

My main motor sources:
https://www.servocity.com/motors-actu...
http://www.lynxmotion.com/c-71-planet...
http://www.robotshop.com/en/dc-motors...

gearboxes for bigger motors:
http://www.banebots.com/category/GEAR...

Need more brushed motor theory? https://en.wikipedia.org/wiki/Brushed...