As a props person, you’re always looking online for images, whether as research for pieces, inspiration for set dressing, or as elements for a paper prop.
The New York Public Library has a great digitized collection of images in their Digital Gallery. Some proptacular highlights include cigarette cards from pre-1900s to the mid-20th century; dust jackets from American and European books, 1926-1947; decoration in the Age of Napoleon; and a collection of restaurant menus from 1851 to 1930.
There is also a ton of theatre-related photographs and ephemera, such as the Vandamm Studio Photographs of theatre productions and players from 1900 to 1957.
These digital galleries have so much visual reference and research; I highly recommend taking the time to look through it all and bookmarking or taking note of what you might find useful in the future.