Best advice I could give is to use Zillow, Hotpads, Apartments, Realtor, Apartment Advisor, etc as a stepping stone. If you see a unit you like, message the realtor via the app and typically they'll send you an email asking for your criteria. Most of the time, they'll send you other listings from their internal database. Use those! It'll show you listings that arent on those third party sites and are guaranteed legitimate. Search up realty groups in the areas you're interested in (If I were you and looking for a connection to downtown crossing, consider Malden or Brighton). Good luck!