It's a startup that does online booking for transit advertising. I assume Radio FM advertisers and newspaper advertisers as my target audience. So, how can I reach them? Do I contact advertising agencies or directly reach brand owners/marketing department?
Depends on the actual content of the ad. If you don't do the ad yourself, the best way to go is to get an agency so they will do the ad creativity and placement.
Needless to say, that's by far the best option. You don't want to end up paying for an expensive ad that is not converting the right amount of people.
What I've done is reach out to agencies that are behind my favorite ads in the medium, and start from there.