You will achieve more by contacting agencies directly. Every larger public library has the Standard Directory of Advertising Agencies. It has a geographic index in the front, showing all of the agencies in (for example) Atlanta. Make a copy of that page right at the library. Then look at the individual agency listings in the body of the directory of the agencies in Atlanta. Each listing shows name, address, phone number, number of employees, billings, roster of clients and names and titles of officers and managers.

Make a copy of each listing of interest. Then send the agencies your resume with a cover letter. Address it to the Human Resources person if such a title is shown. If not, send it to the President or Chief Operating Officer. Make your letter "stand out" (they get hundreds each year) by describing why you feel you are well suited for a career in advertising.

Good Luck!