I’ve had this on my links page for a while, but since I am once again rediscovering its glories, I wanted to call your attention to the Notman Photographic Archives at the Musee McCord, which you can view online via their Keys to History. This is a collection of over 450,000 photographs from 1860 on (plus a few from 1840-60), which just bazillions and bazillions of fabulous period portrait photography. Like this beauty. Go! Search! Download!