The Tate Museum as digitized all the pages from a series of Turner's Sketchbooks. You can view them chronologically here: http://www.tate.org.uk/servlet/BrowseGroup?cgroupid=999999995
In case you are not familiar with Turner, he was an English painter and printmaker. Here are a couple of my favorite of his paintings.
|J.M.W. Turner, Rain, Steam and Speed-The Great Western Railroad (1844)|
|J.M.W. Turner, The Slave Ship (1840)|
Jasper Johns (Born in Augusta, Georgia)
|White Flag, 1955|