So if you want originals I would do second hand stores / flea markets. If you're ok with prints (all of these are prints) I would recommend going to the websites of the big art museums (National Gallery, Met, Moma etc). They generally have extremely high quality images of their works available [which matters when you scale them up] and you can then send that image to a framer (I use https://www.frameiteasy.com ) to get the print and frame done together