Here's the most recent site I've found with a wealth of public domain art: the Victorian Trade Card Collection at Miami University.
These beautiful business post cards are a trove of all kinds of art. Covered wagons in Nebraska advertising Arbuckle Bros. coffee. A view of Ohio's Garfield Monument, the presidential resting place, brought to you by Mica Axle Grease. Tobogganing children to promote Champion-brand biscuits.
The site explains that tradecards were a uniquely American form of advertising that reached its zenith in the 1880s and 1890s. Thanks to Miami University (that's the one in Ohio - not the similarly named university in Florida) there are thousands available online.
