Hello! I live in Boston. The closest place to Boston that stocks N scale is Charles Ro supply company (the retail store for G-scale manufacturer USA Trains) in Malden, MA. They carry most scales and have a decent variety.
About 50 miles from Boston is Shepaug Railroad Company in Leominster, MA which carries a large variety of N scale stuff as well as HO (the owner is an N Scaler himself!). They will be moving to a new location at the end of this month but will still be in Leominster, ma.
Places to avoid:
South Shore hobby is in East Weymouth, MA and they're open whenever the owner, who is an elderly man, decides to open. I've never been there when they are open but from what I've heard, they generally carry antique HO steamers.
Minuemen hobbies in East Braintree, MA has a few HO trainsets but generally caters to he R/C crowd (most likely does not carry N scale)