Part of job board marketing includes email drip campaigns to your target market of employers. If you are starting from scratch there is a service called LeadCandy that can help you get started.
Simply tell them what industry you want to target and ask them to create a list of Talent Acquisition Managers (or other titles like HR Manager) and they will build you a list with all their contact details, LinkedIn URL and more. Typically ‘talent acquisition’ is usually responsible for job board spend vs HR but it can depend on the size of the company. In a small company the HR Manager may do it all.
I have used LeadCandy for many years I can have been happy with their reliable lists for prospecting.
In addition to TA & HR professionals, you could potentially have them build lists of job seekers based on industry or job title.
A custom list has cost me about a $1 per email but they do offer pre-built lists for TA/HR for pretty good rates.