The Wellfleet Oyster guy is at the Davis Square Market today. I already got mine so no one has to worry about edging in on my stash. This guy is great. Please buy from him so he'll keep coming back.
PS - I asked him for certain items today (chowder clams) and he said that he would try to get them in next week. If you go, ask for what you don't see there and maybe there will be a wider selection next week.