Peter Askew who runs RanchWork.com said on Twitter he tried some ads on Reddit and had some good results. Might worth a test to try it for your job board. Go to Reddit Ads.
Job Board Ebooks: https://gumroad.com/jobboardsecrets
My Concierge Service can get you launched in a week or less. Details here.
Using off the shelf software you too can be a job board owner!