Why would you try scraping the bottom of the barrel with backlinks as a service, when you could sell ad spots on a directory in my opinion. I dont know if your site is broken up by niche or geo, sounds like niche though. Sell spots at the top of the page or make a preferred vendor list/page/option. B2B marketing is pretty unrestricted when you think of it compared to B2C where you cannot email in a lot of cases without permission or call them do to the DNC registry. Hire a VA to do some cold emailing to people in this niche.