I recently hired a heating contractor after my Weil-Mclain boiler kept setting off my smoke detectors. It was kind of a frustrating experience -- I never think about my furnace unless something goes wrong, and as a result, I don't have an HVAC contractor that I consistently use.
My first reaction was to call the company that sells me my heating oil. That worked out fine, but it got me to thinking about how I would find a contractor if I had to replace my boiler, rather than just getting it fixed. It seems to me that the key thing is to get estimates from multiple contractors. Ideally I would get estimates from several contractors that were recommended to me by friends and neighbors. But, if I couldn't find enough contractors that way, I would look online. Here are a few heating contractor resources that look promising:
Good luck!