Fishmongers or the farm direct: tightly closed, heavy with liquor, stored cup side down.
Worth knowing
Shuck with a proper oyster knife into the hinge, cup side down, with a folded cloth over your hand. Cut the muscle underneath, keep the liquor and check for shell. To cook them, warm them no more than a minute — poached in their own liquor, or under a spoon of butter on the grill.