Advice for Building a List of Contractors and Sub Contractors

[Matthew O'Hara]

I am an agent in Ma working exclusively with investors. Some of them are fairly new to the process or even doing their first flip. I'm trying to be a better resource for them by building my own list of contractors and subcontractors to refer to clients. I'm not searching for referral fees I just want to make sure I'm helping my clients prepare for their projects and I want to make sure I'm referring them solid potential contractors. Any advice for building a list? 

So far I'm messaging some people on facebook and trying to get on the phone with them to explain what I'm looking for but any other potential approaches would be helpful.