When researching the law of nonprofit organizations, keep in mind that you may be researching state and/or federal law depending on the issue(s) you're searching.
Organizations obtain nonprofit status from states, and state law governs the formation and governance of nonprofits. (The corporate form may also be called "benefit" or "nonstock" corporations in some states.) In contrast, the federal Internal Revenue Service (IRS) grants tax-exemption, although organizations may also apply for exemption from state and local taxes as well.
Also note that while most people think of charities when they think of nonprofits, charitable organizations are only one of the many kinds of nonprofit organizations that can request tax exempt status under section 501(c) of the Internal Revenue Code.
Lexis and Westlaw do not have nonprofit law as a practice area, but both have helpful practice materials related to nonprofits (West: Practical Law, Lexis: Practical Guidance). Both also have a number of helpful secondary sources, noted throughout this guide.
Many trade news and scholarly articles on nonprofits are found in the business and public affairs literatures. Some of the largest databases for that literature are listed below. UCLA Anderson School of Business Rosenfeld Library has a list of more business databases. (Note that some are only available to Anderson students.)
Articles about issues in specific segments may also be found in other databases (e.g., GreenFile for environmental nonprofits, Pubmed for healthcare nonprofits).
Historical English law sometimes has an influence on US charitable nonprofit law. This is due in large part to the continuing impact of the Charitable Uses Act of 1601, also called the Statute of Elizabeth. For help researching English legal sources, see the UCLA Law Libguide on English Legal Materials. For case law, Yale Law Library has a useful chart that breaks down which reporters covers which years.