A lot of art from Sierra games is in the hands of former employees and in the private collections of fans who have acquired the art. There’s no “complete set” of art for any one game that is in the hands of one person.
“The Art of Sierra” project is continually looking for any and all final and concept art, development materials of any kind, photos, promotional materials, archived interviews and articles, and any other items related to Sierra – any Sierra-related items are considered valuable to preserving the history of Sierra for all fans to enjoy.