It's a "concept pitch". Give them the entire idea and persuade them that, while it's a start up, it will quickly become a great source of driving business for their real sites. They need an incentive. I doubt you're prepared to provide a monetary reward so convince them that this project will become a powerful website and they're getting in at the ground level.
-Post on Craigslist
-Hire from guru.com
THose are two good ideas.