Ask HN: Where would I get a list of Mechanics/Docs

We recently started a service for mechanics/docs/vets. The site is done and all, but now comes the hard part of gaining traction, what is the best way for us to get mechanics and doctors and such to sign up to our site. I am thinking if we can somehow get a list or contact some sort of large publications that cater to these professions, it might work. So far we have: done a facebook campaign (got likes, barely and signups to our free service), tried yellowpages cold calling (no go), tried twitter (no go), tried emailing bloggers and webmasters of sites in the field (no go), got some small time news press (barely any go). For example, how would a online tutor company recruit these tutors to sign up and scale? any ideas? We launched a 3 weeks ago, so I am sure we have some other avenues, and we know there is a market for our service because our competitor is doing very well.