Sam Adams has two different beers: Boston Lager and Boston Ale. They share the same malt bill, but one is made with their house lager yeast and the other with their house ale yeast. For the Boston Lager, I would check out one of the American lager strains: WY2035 or WY2272 (sorry, I am really only familiar with the Wyeast strains). If these were not available (as both are private collection strains and only released occasionally), I would look to a clean lager strain such as WY2206, WY2124, Saflager W-34/70.