Aperture Foundation Gallery:
George Eastman House:
International Center for Photography:
National Geographic's Photography Page:
Society of Photographic Education:
Women in Photography International:
The "google" of government documents--simply type in "photographs" to access government images:
Historical collections that exist in digital format:
Interlibrary Loan is a free service that allows you to request materials to be delivered to your campus library. There are two options for requesting materials: requesting an item directly from one of the Library Catalogs or using Interlibrary Loan Request form.
Be prepared to wait at least several days for an article and at least a week for a book.