It is difficult to get Boston parts from Steinway. Sometimes Pete at Steinway has told me they can get X but it will take a couple months. A few things they have in stock. When I asked Pete about how others deal with getting parts, he said he assumes people get parts elsewhere.... I guess that means they don't mind you putting non-Boston parts on their Boston pianos.
Many times if I email the parts department at Steinway about a Boston part, I don't even get a response.