Reddit reviews Motors for Makers: A Guide to Steppers, Servos, and Other Electrical Machines

Others friends have used:

Mitsumi - super common, affordable, not necessarily high-end

Maxon - Swiss, highest of the high end, used on Mars robots, etc. $150+

Weird Drone Racing Motors - haven't heard of any of these guys but you can tell they're eaking out every ounce of performance

Tamiya, Kyoshi, Goki and other Japanese model companies make some great motors. Maybe not for mass production.

Global Sourcescan be a good source for vendors - I see "Sinopro" comes up a lot.

Last but not least, I liked this book: https://www.amazon.com/Motors-Makers-Steppers-Electrical-Machines/dp/0134032837

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This book is a good overview.

https://www.amazon.com/Motors-Makers-Steppers-Electrical-Machines/dp/0134032837/ref=asc_df_0134032837/

Some book suggestions:
Making Things Move
Physical Computing
Motors for Makers

These books can be found on the Web ...

Maker Media, Inc., publishes many books and a magazine that could be of inspiration.