Try a Google search for [Company Name] AND mission
You can substitute "vision" or "values" for mission if that information is not provided on the same page as the mission.
Preference should be given to results that are drawn from official company websites.
Some companies use mission statement-like language, even if it is not labeled as a mission statement. Try finding the company's corporate or investors webpage. Again, search [Company Name] AND corporate, or [Company Name] AND investors. This information is often present in an "About" page
You can also try contacting the company's Public Relations or Marketing Departments as a fallback plan.