There've been several threads on the search for New England-style hot dog buns:
I've found them at a place in Monrovia called the Market Grill that opened last November. I believe the owner is originally from Lowell, Mass. They bake the buns themselves. They're excellent, but unfortunately the dogs themselves are disappointing. The manager said they would sell the buns separately if I called ahead of time, but I haven't tried it yet. Note that they sometimes run out of buns early.
BTW their fries are outstanding -- better than Oinkster or Wurstkuche in my opinion.
The Market Grill
525 S Shamrock Ave, Monrovia, CA 91016