Perhaps the job was never filled or the person previously hired has already moved on. The companies themselves . Call the human resources or public relations department of the firm. Get brochures, an annual report, descriptions of relevant jobs and anything else that describes the company. Meet with someone from the firm to get more detailed information about the company itself and possibly a job lead.
Most industries have their own trade associations.
These associations may hold regular meetings and publish periodicals, both of which are good sources of inside information about member companies. Many professionals belong to one or more professional associations. If you never joined yours, or your membership has expired, this might be a good time to get active. These organizations often have a membership directory, which is an excellent source of names for networking. Professional groups usually have regular meetings where job openings may be posted.