[mothered]

The registrar of the domain will give me the contact info?

Registrars take their user's privacy seriously. Without any form of credentials, they won't disclose any Information.

Check the domain name against all social media. Do the same with any keywords of relevance from the website and also perform a deep online search. The objective Is to Identify the association of an online account (example, Facebook, YouTube, Blogger, LinkedIn etc) with that of the website. Once any part of that has been established, you'll either have a starting point to formulate the contact details, or have the Information In full.

In short terms, do lots of recon.

When applied correctly, It can be very effective.

I've built a complete (user's) profile from the ground up, with only a cell phone number at my disposal. It's not only about how you collect Information, but also the methods used to formulate your objective.