They are available at M.I.T. and the Somerville, Waltham, Watertown, and Woburn Public Libraries (not the Boston Public Library). There is no remote access; you must go to the library. The maps were digitized from the microform set and are in black and white.
Sanborn fire insurance maps Massachusetts [1867-1950]
Black and white. 48 microfilm reels.
Microfilm of Sanborn Map Company maps held in the Geography and Map Division, Library of Congress.
Mugar Library Microforms HG9778. M37 S45.
Boston, 1867, v.l: Downtown; 1885, v.1 Downtown - South End/Sections 1-25C; 1887, v.2 Back Bay - South End/Sections 29-59; 1888, v.3 Roxbury - Jamaica Plain - West Roxbury, parts of Brighton & Brookline/Sections 62-100; 1888, v.4 South Boston-Dorchester - Mattapan/Sections 106-141.
Boston, 1895, v.1 Downtown - Chinatown/Sections 1-58; 1897, v.2 Back Bay - South End - North Dorchester/Sections 1-83; 1897, v. 3 Roxbury - Mission Hill - Jamaica Plain (West Roxbury)/Sections 1-112; 1899, v.4 South Boston - North Dorchester/Sections 1-104; 1900, v.5 Charlestown - East Boston/Sections 1-111.