If you're looking for people who will be looking for events to sponsor, I would start with the events themselves: who is sponsoring them? Contact the event organizers to get in touch with the sponsors, and tell them that you want to help them find more events like this one to sponsor in the future. Did you try that tactic yet? I'm actually building a product for marketing executives as well (solving a completely different problem) so I'm happy to chat and exchange notes if you're interested.