My most used source for quality motors, controllers, and converters.
Herbach & Rademan
I have found them to be pricey sometimes, but they deliver a good product, or did so that last time I bought from them.
Occasionally, American Science and Surplus carries usable stuff too, but they must be watched as far as "WHAT" they are selling you. Sometimes you are not getting what you might think you are. Caveat emptor!