Metropolitan Museum of Art
Metropolitan Museum of Art: John Singleton Copley
This sample "exhibit" from the New York Metropolitan Museum of Art offers a short biography of colonial painter and portrait artist John Singleton Copley. Includes links to six different paintings, each with an informative caption.
Other
Brock University: Some Notes on Realism
This site talks about the different aspects of 19th-century realism. (From realist painting to literature)
Other
Electric Gallery: Abstract Realism Collections
This site includes the works of many realist artists. However, these paintings are more modern "abstract realism" rather than 19th century pieces.
Other
Richard R. Gandy Gallery of Realist Art
A gallery by Richard R. Gandy of Classical Realist art. Includes a good list of artists and has good images and information about works of art.