The IRS defines a 501(c)3 organization as a non-profit or charitable organization.
The organization "must not be organized or operated for the benefit of private interests, and no part of a section 501(c)(3) organization's net earnings may inure to the benefit of any private shareholder or individual".
Here are the Exemption Requirements.
The Foundation Center has an online directory of private philanthropic foundations located in the United States. Many libraries also have print copies of the Foundation Center directories.
You may search the Foundation Center database for Form 990 filings (Return of Organization Exempt from Income Tax). To learn more about the 990 form, click here.