The following three databases cover many different subjects, but none in great depth. Very good for retrieving materials from other disciplines.
Nonprofit organizations are not subject to the same guidelines as publicly-held companies, but their tax exempt status is regulated by the federal government's Internal Revenue Service under IRS Code, Section 501(c), so there are some reporting requirements. This information is intended to help people make good donation choices, but you can also use it for a number of research purposes.
Some information like the 990 may be available on the organization's Web site (if you have one identified), so you can start there, but the following sources will provide a more complete picture.