If I were doing this, I’d spend some time (but not too much!) building the list, then make a multi-front approach using calls and emails.
Once you have the list, what do you plan to say to them? Will’s graphic he posted pretty much says it – what thing do people in that business complain about? How does your product help with that?
Hint: Your own support/service backend knows what the most common complaints are with your product. Odds are, your competitors products have the same or similar weaknesses. That might give you a starting point to develop your approach.