Finding out if company founders are graduates for a dissertation
The most reliable source is the college/university itself. You can compile a list of where these founders when to school, and call the school itself and ask the question. The question has to be Yes/No so you will have to phrase it like: "Did you award such-and-such degree to John Smith?" 

If that is too labor intensive, a simple Google search of the person "John Smith College" should return results from reputable sources such as Time, Business Insider, etc.