The "big" job boards — Monster, Careerbuilder, etc. — have had their heyday … and it's over.
Job board aggregators are still working – Indeed.com is probably the best of these.
The SmartRecruiters blog has collected 50 of the "best" niche job boards and sites here.
Just keep in mind that only 5% of jobs are found online. So you can share these niche job boards with clients, but make sure to emphasize that 95% of jobs are NOT found online!