Is there a site where freelancers can post a bounty for new projects?

There are plenty of sites where freelancers can bid on existing projects. But does anyone know of a site where freelancers and small agencies can put a bounty on referrals to new gigs? For example -- I run a small web shop and would pay generously for a referral that leads to a good project. Say, 10% ??

I've got an idea for a new website and I would like to validate it here on HN and in other places...

lets discuss...