I use some well established archives online; such as the Library of Congress, the National Archives, Getty Images, that sort of thing. Most of those sites will have information regarding the photography, context, dates, etc, in addition to the photo itself. Some will be watermarked so I look for the image elsewhere online. Within the sites, I search for subjects of historical value, some might be little known historical events, cultural trends, big news, pretty much anything that's grabbed my attention prior to looking things up. It's all random and a crap shoot what I end up posting.