I'm always so sad seeing the Scott expedition photos knowing that these guys in this image were doomed. THe British crew would have felt so horrible spotting the Norwegian flag at the South Pole, after what they went through. I've read the accounts and went through these beautiful images, it's so harrowing to know their fate.
I seem to remember they found Scott's notes, as well as documentation, including photos (plates?) of them at Amundsen's abandoned tent when they found the bodies but can't find a source just now.
Mixture of places. Most I get from Rarehistoricalphotos. Many I get from Wikimedia commons. Some I google, when I have a piece of historical trivia I know but no picture to go with it. When I feel adventurous, I search online national archives, found some good Ethiopian pics in the Polish archives a while back.