I was wondering whether there are any makers/sellers of indigenous folk instruments along the way. I have an interest in the music of the medieval pilgrimage and I'd particularly like to come home with a set of bagpipes or some other reed instruments.
Try asking a friend of mine, Dirtyshirt Jake. He runs an Irish music school in Valencia and his wife is from Santiago. He should be able to lead you in the right direction. (Tell him "Wobber" sent ya...)
(Not that you can buy them, but since you have an interest in that area...)
When you're in Santiago de Compostela you might want to visit the museum of the Cathedral (it's accessed from Prazo Obradoiro) . There is an exhibit there with replicas of the instruments that the various figures carved in the famous doorway are holding.
I think if you keep asking as you go along, you will find what you're looking for. Of course it would be good to help your own cause by learning the necessary vocabulary in Spanish if you don't already know it.