as stated already...3 euro bus from Plaza Galicia
If you need assistance locating it or anything in Santiago, just go to the tourism office, which is very near to the cathedral. They are very helpful there and gracious and they will also stamp your pilgrim's passport.
Peaceable Projects Inc. is a U.S.-based non-profit group that brings the vast resources of the wide world together with the ongoing needs of the people who live, work, and travel on the Camino de Santiago pilgrim trail network in Spain.
So last year on my way out in a taxi I realized I left my last remaining m95 mask on a table outside my hotel near the Cathedral. Nice taxi driver drove me around Santiago until we found a farmacia open. He pulled to the curb on the third attempt, I came back with new masks.