Some free sources to find owner information (not list building - getting lists can be a combination of paid tools like Apollo and free resources like Google Maps, industry associations, LinkedIn, etc.)

1. OpenCorporates.com (free website that aggregates state/gov filings for registered company owners. Similar to other websites like BBB but IMHO more comprehensive)
2. TruePeopleSearch.com (free website that also aggregates state/gov filings for personal cell phone numbers)
3. Bounceban.com (email validator tool - their dashboard has a free email validator with unlimited usage)
4. Connect with them on LinkedIn. In addition to being able to message them on LinkedIn, you can also often see their personal email by clicking on "Contact info"
5. GetCohesiveAI.com (shameless self-promotion - freemium Google Sheets extension that can guess/validate owner emails among other things)