Whois lookup gives you the first point of contact of domain ownership. These days, most will sit behind an anonymizations service but you may get lucky and find out contact info there. If they are anonymized, and you have a legal claim, get in touch with the legal contact shown there.

If it's a relatively established website, they likely have a trademark on the name, so the next point of call would be USPTO or WIPO to search for that trademark. That usually gives you non-anonymized data.