All good suggestions from howard. Although the big boards have been useful in the past, online recruiting has largely migrated away from the Careerbuilders to more specialized websites. CB tends to be expensive and rarely produces.
Do a bit of homework first. Find out who the competitors to your client are, and certainly those that may be in the local area. Then do a LinkedIn search on those competitors. You might get very lucky and find someone immediately, but chances are that you'll just get some names from those competitors you can call and get referrals from.
Most professional associations have job boards, often free or at nominal cost. These will give you a very targeted response, not the overwhelming deluge of unqualified resumes you'll get from CB or Monster. Many also have a searchable resume database. The Society of Mechanical Engineers or Society of Manufacturing Engineers would be a good starting point.
Local universities will allow you to post on alumni sites. Look for local universities which have manufacturing or mechanical engineering degree programs.
Also check out outplacement firms such as Lee Hecht Harrison or smaller local ones. They have free job posting boards and resume search for their outplacement clients.