From your post it’s unclear whether you’re trying to hire an agency to help you, sell them your product, or simply get an informational interview for conducting research….
If its hiring an agency, then money talks. “Hey Mr. Ad Guy, I’ve got some marketing dollars burning a hole in my pocket.” You’ll be up to your eyeballs in martini lunches.
If it’s selling something, then see the next bit…
The next bit: The best approach is, “I need to do some research, can you hook a brother up?” Or you can be all business like and say something like, “We need some help validating our idea and thought you would be perfect to help us. Can I take you to lunch and ask a few questions?” You get my point.
Ad execs and creatives love to give advice. Heck, everyone loves that. Asking for advice gets you in the door so to speak. Other posters have mentioned awesome places to meet these peeps. I concur. I would ad social media as well. Find where they hang out digitally and ask for their advice. This often can and will lead to a sale if that’s the ultimate goal…
Keep it simple.....looks like you an offering for Agency and their Clients, not that you are looking for an Agency. The best success that we have had with start up or early stage companies has been by showing "proof of performance". Work with a Brand and show the success there. Once you deliver on that, you identify the right Ad Exec that has the rolodex and from there, you will be on your way.