You could start with LinkedIn (Profiles and Groups). Search for Marketing Executives and join the relevant groups. For example, this group http://linkd.in/1a8zR3c

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.

Google Consumer Surveys are very low cost. Although they are designed for B2C rather than B2B, you can ask qualifying questions, and it may be a way for you to get responses from marketers.
http://www.google.com/insights/consumersurveys/home

Ohh I love this question!

I've done this a bunch of times to get in front of VP's and C-Level executives at F500's.

Create some sort of publication. Maybe it's a blog, maybe it's a podcast, it doesn't need to be fancy, it just needs to be real.

Create the topic that you're interviewing them about. Let's call it, "Future of Marketing: 2013 and Beyond". Your pitch is that you're interviewing top marketers to see where the future of business lies. You only want the smartest.

Go on Linkedin, search the title you want and get some inMail credits (these are really effective).

Begin reaching out for interviews for the magazine/blog/podcast via inMail.

You'll need to get past their PR peeps because they will want to make sure you're going to make them look good.

And having a list of names of other people you're about to interview that are high-level helps.

You'd be amazed at how easy it is to get to these guys via media.

Happy to discuss exactly how to execute it, create the site, the pitch and even the email templates to get you in the door on a Clarity call.

All the best and have fun!