adflip [fee-based but very reasonable rates for short-term access]
ADVERTOLOG [subscription-based, but personal, reasonable, short-term accounts possible]
Coloribus Advertising Archive [also subscription-based, but personal acct. feasible]
Global Ad Source [fee-based service]
Also see useful links in Advertising Guide from Eastern Michigan University Library
Ad Age On Campus -- Digital version of Advertising Age, with access to Ad Age Datacenter.