A good first step in your research is to determine if a company is public or private. A public company is usually traded on a stock exchange and as such is often required to provide detailed financial information and more.
On the other hand data on private companies can be difficult to come by - particularly in the United States. Good sources of information for private companies include news articles and incorporation documents. If you can determine where a company has incorporated the State Department for that state will be able to provide you with documentation. Another good source are credit reports - these often require a fee.