It can be challenging to find detailed information about private companies. Services that offer information about private companies compile it based on publicly available sources, and may augment it through modeling or projections. You may want to consider this when choosing a company to research for a project. 

Here are sources that include some private company information:

Business Source Complete includes reports on large companies, including some private companies. 

Experian Business Reports offer analysis of a company's debt that may include how much they owe, how big their credit line is, their payment history, and what lines of business they are borrowing for.

Also search LexisNexis Academic and LibrarySearch for coverage of a private company in news or in trade publications.