Finding information regarding privately owned companies is more difficult than for public companies since the government does not require private firms to file the many different financial disclosure and other forms that it requires of publicly traded entities.
There are nonetheless resources you may use for your research. An obvious one is the company's website, though always keep in mind that the primary purpose of the website is PR for the company rather than providing unbiased information for researchers.
In the US, private companies are not required to release their financials. Therefore, most of the time you won't be able to get detailed financial statements on a private firm. Here are some strategies on researching a private company:
Library of Congress Guide on Private Company Research
The following library resources are some of the best at providing objective information: