Companies that produce robots (and other CNC equipment) invest a lot of time and resources into their development. They not only won't be providing their design documents, they are likely to take legal action against anyone that reverse engineers their equipment and publishes it.

There are some free 3d printed desktop robots available on many 3d printing websites. You could download their models and measure and copy them. But like the professional robot engineers, they won't be providing the documentation for free. Be careful about reverse engineering one and then publishing your results. You would be violating somebody's copyright. I know if you were to copy and publish one of my designs I would be doing everything at my disposal against you.

You're best bet is to learn from what someone else has done but design your own completely from scratch.