This site is my long time favourite, this is a fantastic colections of ads, illustrations, covers and much more!
http://designarchives.aiga.org/#/collections
AIGA Design Archives is one of the richest online resources available to those who practice, study and appreciate great design. It represents the quality of work being created, as well as shifting aesthetics and sensibilities of the designers of the day. Included in this resource are more than 20,000 selections from AIGA’s annual juried design competitions dating from 1924 through the present.