Robots are a bit different than RC in that it utilizes a wide variety of electrical and mechanical components, many of which were not originally designed for robotics. It's going to be difficult to find a big superstore.... For example, Digikey and Mouser have millions of electronic components, whereas McMaster-Carr or Stock Drive have hundreds of mechanical components. You are best served from buying from a group of companies... My favorites are Sparkfun, Parallax, Pololu, Jameco, and
Trossen Robotics, just to name a few.