We would like to produce a regular email newsletter but don't have the time to do all of the content creation ourselves, so we thought of commissioning regular content, and adding a few of our own articles as time allows. Junta42 is the only marketplace I've seen so far; has anyone used this or a similar marketplace?
There are plenty of job boards out there, why don't you just hire someone to write content for you?
You could look at sites like Elance or Odesk to recruit a contractor to write regular articles. A service I've been using lately is MediaPiston. You don't necessarily get the same writer all the time (though you can try and request your favourite writers) - but you get content easily and can request changes before publishing. Worth a look at least. Their site is www.mediapiston.com.