Try one of these three, “China” stores:
The largest one I know of in Santiago is at:
Rúa de Alfredo Brañas, 18, 15701 Santiago de Compostela
It is on the right side as you are heading away from Rua de Montero Rios... It is split into two shops in a “V” shape.
My personal favorite, albeit smaller, is on Rua de Montero Rios, directly opposite the Carrefour market.
The only one in the old town, but smallest, is at:
Rúa da Caldeirería, 58, 15703 Santiago de Compostela
Hope this helps.