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 informatioan 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.
► International Directory of Company Histories
Via Business Insights: Essentials
Also available in print: HD2721 .I581988 (Reference Stacks)
Contains a 2 or more page summary of a company's history along with a brief bibliography of other sources for information. Companies included must have at least $500 million (US) in annual sales or be a leader in their industry.
► Notable Corporate Chronologies
Via Business Insights: Essentials
This source identifies important events in a company's history.