Its a very tough thing to find someone who will do quality work on a commission only basis. I would look affiliate marketers and try to groom them to do some extra work. In the past i have hired these "personalilty" types in person when i met a good sales person at Best Buy or the Ford Dealership.
You can also try craigslist, but the problem is that craigslist is over saturated by millionaire mom opportunities.
If yours is a web app, i would consider Facebook ads. Although Facebook ads are horrible for most situations, you could use them effectively by targeting those who are members or groups or are demographically friendly to your target sales people.
Also consider giving them a base. A base allows you to attract a higher caliber of employee.