Where to get the scallop shell?

Ask around in Ferrol in the cafes where they sell sea food for 'vieira para peregrino'. Probably best places to try will be near the harbour.
Buen Camino

Surely the point with the scallop shell is that you get it once you arrive on Santiago as a symbol of completion.

Tradition and convention suggest that the shell was worn by pilgrims who had completed their journey to Santiago and were on their journey home. For some pagans the rays of the shell reflect the rays of the setting sun as it sinks in to the sundering sea. The wearer has returned from the end of the world. Recently the wearing of the shell by those beginning their pilgrimage has become established as a symbol of commitment, or perhaps just as a tribal icon. There seems to be a growing convention of gifting (or selling) a shell to pilgrims as they pass certain places. There is a bar/restaurant in Neda where the Patron signs scallop shells for anyone with a pack or boots. StJdP has enough red-ribboned scallop shells on offer to challenge the souvenir shops of Santiago.

Curiously virtually all these souvenirs are fashioned from the flat side of the scallop shell yet it must have been the cup shaped half that Santiago used to scoop water and slake the thirst of the pilgrim tempted at the Fuente Reniega on the slopes of the Alto de Perdon. The concave shell has been a useful tool since neanderthal first cooked scallop a la mode.

With icecream?

a la mode de maison (trans: the way we do it round here)

Yes...i know.