Finding out the names of c-level execs is usually easy whether from tax returns (if a non-profit) or financial statements (if publicly traded). Then its just a matter of figuring out what configuration they use to make their e-mail addresses, which you can often find out by finding the e-mail of anyone in the company (e.g. (first initial)(last name)@company.com or (first name)(last name)@abbreviation for company.com, etc.)
(Some c levels have caught on to this and have added an initial or something to break the code, but its worth a shot)