You won't find many artist's multiples in Artstor, but you will find a few examples by searching for "artists' multiples".
Try these other image collections/exhibitions of artists' multiples, or search the web for: artists' multiples exhibition:
Here is a list of places to buy/sell multiples:
Sherrill Library | Lesley University, Brattle Campus | 89 Brattle Street - Cambridge, MA 02138 | 617-349-8850
John & Carol Moriarty Library | Lesley University, Porter Campus | 1801 Massachusetts Avenue - Cambridge, MA 02140 | 617-349-8070