Where can I buy an excellent Boston Cream Pie
Quebrada is delicious and classic style, but I think you may have to order ahead or at least call - Arlington is the closest to Cambridge.
Flour's version looks awesome but I'm reluctant to try one that includes coffee syrup in the menu description - doesn't seem right to me.
The one served at Kingfish Hall is also excellent (really rich, like most of their food), but I don't know about getting a whole one to go from them.
Don't know about retail, but I've had versions I really liked at Flour (South End) and at Novel, in the Christian Science Center. The Parker House invented it, but truth be told I haven't had their version in a long time. I think the Oak Room serves it, probably on a silver salver. Expect touristy places like Durgin-Park to offer it, too, though I couldn't tell you if theirs is any good.