I think you can get lobster rolls all year. B&G Oysters in the South End has my favorite version. Kingfish Hall in Faneuil Hall has a pretty good one. Neptune Oyster in the North End has a hot buttered version and a traditional one as well, though it's more mayo-ey than I like.
Thank you for the response, Lissy. I have some more questions which I hope you might find time to reply to.
What constitutes your favourite version?
From my limited research on Chow and other sites, I understand there are 2 camps of lobster roll fans: mayo and butter. I don't like mayo much. Can I ask for the butter version at any of the places you mentioned?
What is the average price?