We are a private detective agency and one of the services that we offer is mobile fingerprinting. Our technicians go to the businesses and fingerprint the employees. Most of our clients are insurance companies, financial services, non-profit organizations and governmental agencies, to name a few.
I think there are two parts to your question:
A) List Building
B) Engagement of your list once built
On the list building side, I would suggest you develop a strong content marketing strategy. With a B2B audience, there are really great affordable solutions like HubSpot that can give you the tools and tracking you need to get a solid ROI on any content you create.
When thinking about content strategy, its rather basic: consider how your business and industry knowledge can be turned into white papers, trade-focused blog posts, downloadable e-Books, webinars, etc. that would offer value and utility to your potential customers. Create that. Then, use SEO, social, and native ad services like Outbrain or Taboola to drive sales lead through that funnel. Make sure the landing pages you create have a strong call to action that includes providing an email. From that point, you can shift to keeping those leads engaged, and driving them through your buyer journey funnel.
Other ways to build your email list can include doing a giveaway when you're at a trade show, doing free webinars that help and ads value to your client decision makers (and require email opt-in). You can also ask related businesses in your industry (non-competing ones) to send emails to their customers (co-branded) inviting them to the resources or webinars you've created. All that to drive traffic to email opt-in windows and build your list.
For the second part, once you have a list, content marketing can play a key role. I assume you know who are the key decision makers on your client's side of the business. Develop content that is valuable and useful to those sales leads, and use email to deliver that content. Again, this is where HubSpot and other tools like theirs can help you outline the strategy and the tactical plan.
I would also highly recommend using services like MailChimp or aWeber to maintain consistent email creative, and to follow email marketing best practices. Their websites are also valuable resources to help you become email marketing JEDI.
Whatever you do, I would highly discourage you from buying any third party lists, and would strongly encourage you to always follow opt-in best practices--even if this is a B2B email campaign. There is no long-term profit in spamming your potential customers.
Hope this helps!