First question you should ask yourself:

What value am I bringing contributors?
If you are going to pay them (much like this site) for their time it's an easy sell and you should make a hit list of the most asked questions (80/20 rule) 80% of revenue will come from 20% of your requests.

Then make a list of experts in those fields you find through researching the web and then offer them % of the revenue to answer or help with the questions.

If you're asking for where they are the answer is on all social networks (if their worth their weight in GOLD).

If you have customers asking a lot about a certain service
i.e.
mobile interface design

It should be really easy to make a list of mobile interface devs who are willing to get paid by you to help others.

If you're trying to sell leads to them, give them a few for free and get feedback from them.

Did they get business from it?
Did these people looking for services flake?

RESEARCH will get you everything you want. It's a lot of work but if it were easy, everyone would be rich with business.