The process of finding a contractor can be very overwhelming. You may not even know where to start. You can start with asking friends and neighbors about who installed their boiler and were they happy with the installation. You can look online for boiler installers in your area or on your local blogs. Once you find one or two installers you like, keep up communication with them. Perhaps you are not ready to invest right now or the price seems higher than expected. Continue communicating and asking questions, even questions like "why is the price so high" or "why are you offering this instead of that". As the customer, you should not be ashamed to question the contractor. Asking questions will lead you to a better understanding of the product you are getting and the type of company you may decide to work with. Sometimes the way a contractor handles your questions is a good indication of how they will handle your project, house, and future complications if they arise.
Heatinghelp.com and brownstoner.com are two great sources in the NY area for references and answers to any questions you may have about the process.
Here is a blog post where Gennady describes the best way to go about finding an installer.- http://www.brownstoner.com/forum/#!/general-discussion:boiler-installer-recommenda
"Do the search on internet. Use keywords like “boiler installer in brooklyn”. Do not look at advertisements results, look at natural search results. Also visit heatinghelp.com “find contractor” section. Look at local boiler installer companies websites. Then go to dob website, a810-bisweb.nyc.gov/…/icenseTypeServlet?vlfirst=Y check if contractor on the website is licenced. Then do the search on candidates, see how many boiler jobs they did recently. Boiler work is a specialty, so you must find contractor who does boilers exclusively. See LAA and permits information regarding clients. Contact clients and ask them about their experiences with contractor and job in general. Make sure contractor offers heat loss calculations and EDR survey. Do not rely on size of existing boiler. You don't know if original installer did not do mistake in sizing of the boiler years ago. Another consideration: do you want just to replace boiler or upgrade it to better one, more efficient, do you consider upgrade heating system in general."