With ezines and newsletters, you reach a specific number of people-their subscribers-who have asked to recieve their publication. This eliminates the worry of spam or sending bulk email, which most people really hate.
Here are some links to databases for ezines and newsletters. You can search these databases to see which publications you might wish to advertise your service or product in. You will always find contact information in these databases, and sometimes you will even find advertising information for each publication.