Dover's online store has new items in their Clearance Center--50% off. I see lots of items here for making art. There are art history books and fine art postcards. There are stickers of Victorian babies and children. There are clip art books and books of images in the public domain. Their gift wraps can be used as collage elements or for decoupage.
I have ordered from Dover online before and the customer service was great and the items were received quickly and without error.