Empirical means research was done by the authors: a study, a survey, observation, a lab experiment, etc. That means, no “reviews of literature” type articles, where the authors tell you the history of such and such.
One obvious step for finding these articles is to Medaille Library. See that part that says “find articles”? USE IT!
You may also use Google Scholar. I really like using this because you find a lot of different things. You can search by author (McCombs and Shaw, when searching for Agenda setting articles for example), or subject. Advance search lets you narrow down the year too.