Like private companies, it can be difficult to find information on nonprofit organizations. In addition to searching for articles and looking at the organization's website, the following websites (mostly free, some with additional subscription content), can be useful resources:
Guidestar is the go-to resource for nonprofit research. The free version provides access to a nonprofit's IRS Form 990 filings which can be great sources of information on a nonprofit and include revenue info as well as money spent on top outside consultants and top highest-paid executives.
Another great resource for nonprofit research (including 990 forms). Lots of great top-level statistics, like how many tax-exempt organizations exist broken out by charities, private foundations and other nonprofit groups.
The Free Management Library is a resource for nonprofits. It offers tips and toolkits related to various topics that are relevant to nonprofits, including Boards of Directors, Leadership Development, Mentoring, Decision Making, Growing Organizations and more.