The highest quality and cost is https://contently.com/ - $200+ per post.

Alternatively, I've uses http://jobs.problogger.net/ job board to post an opportunity, and then create a simple "test" for the writers. If they read the instructions and the content is great, try em' out .. if not, then pass.

It takes time but you can find great people who will blog for $30-50 per post.

Another way - not fast but high quality - is to approach existing blogs in your vertical with smaller audiences ... ask them to guest post or ghostwrite for you .. they already know the subject matter and blogging + if they have an audience you can incentivize them to promote the content via their social channel (by paying social share bonuses).

Hope that helps.