Annual Reports contain a wealth of data on a company.
In addition, you can also find annual reports of some companies directly on the Internet. Simply type in [company name] Annual Report, and you will get a PDF of the annual report complete with Form 10-K that was filed with the Securities and Exchange Commission.
Private Companies are more difficult to find information about and more time consuming to research. The information that is available will be less specific and less certain than information available for public companies. Private company information may also be available in one resource and not another.
When it comes to researching private companies, the first place to look would be the company's web page if they have one. Never underestimate the information that companies publish on their own web sites.