Let's say that you have a great idea, great market niche and you can build a prototype and you need a cofounder who has experience in selling and who is connected. How do you find and choose the cofounder who can drive sales?
In your case, the ideal sales candidate is someone who already has been selling a similar solution and knows your customers.
The challenge is to convince a reasonably successful sales person to join you when all you have is a vision. Dreams don't pay the rent.
Stay clear of freelancers who want to suck money from you. Read http://blog.fairsoftware.net/2009/03/27/im-a-geek-how-do-i-pick-a-business-partner-i-can-trust/ for more on how to tell who is who.
Go to local networking events, and look for someone with a background in sales.
Look at local universities, especially business schools. All of them have sales clubs, and many members have sales experience. Join these clubs or just hang out with their members.
Finding co-founders regardless of whether you are the one with the idea or with the sales skills is a challenging process. Someone said it is like finding your significant other in life. Like Yoav mentioned networking is one of the best ways of finding them or getting recommendation or referrals from your network (Using Linkedin for this works well).
However before you start your process list down what exactly you are looking for in your ideal co-founder. I like to use a list of ideal characteristics along with competencies that I am looking for. This makes my request specific and easier for people to introduce me to prospects who fit the bill.
Best of luck in your search.