The Vintage Art collections are exactly what it says on the tin, collections of vintage pieces of art that have fallen into the public domain and as such, may be freely used by people for whatever reason, such as illustration game books or providing reference for an adventure. Now, while you could possibly find many the pieces here if you wished to spend the time, Unza RPG has done it for you and for a reasonable price you get quite a bit of art to use.
Here are some of the collections available:
Sea of Stars RPG Design Journal 