Are you looking for information on specific nonprofits, charities, foundations, or similar organizations?
The sources listed below are good sources of information. If you are looking specifically for financial data, look for mention of an IRS document called a "Form 990" on these sites. These documents provide disclosure of key leaders as well as the financial position and activities of nonprofit organizations.
The Foundation Center is also a useful tool for identifying grant opportunities and grant-making organizations.
Note: UNI has a subscription to the Foundation Center. The subscription is accessible via the link below.