Museums, auction houses, educational organizations, and even government sites can be a wealth of resources on art and artists. See below for some of Professor Gross's favorites. Also, remember that any resource you use must be cited in your work. When you are working on the web, make sure to document every resource you use so that you may find it again and/or give credit to the proper author or organization.
The Metropolitan Museum of Art
Smart History: The Center for Public Art History