INVESTIGATION AND EXCAVATION
Is your topic have local historical significance? If yes, look to America's Historical Newspapers for sources.
Consider town and state historical societies (ex. Arlington Historical Society)
Consider archives and museums on the web; they often have access to images and primary sources that will serve useful.
*use a dictionary! you may encounter terms you've never heard before.
quotes around a search term keeps terms together; search engine won't search for each term separately (ex. "washington monument")
tilda's before search terms search for like terms (ex. ~washington monument will search for washington monuments, washington monument, monuments in washington, and beyond...