Locating private company information may be a challenge, as they are not required to file and/or share company information. However, they occasionally may file forms with the SEC, such as a Form D, or if it was public at one time, an S1. Check with the Secretary of State's Office for the company as some states require companies to file these documents. If a public company is involved with a private company, there may also be information on the private company in the public company's filings.
Performing an article search for a specific private company, especially in a relevant regional or local business publication, is another method of locating information.