I had to pick up a second credenciale last week in Sarria because mine was almost full. I got one from the monastery at the top of the hill - there is an albergue there too but you don't need to be staying there. We just wandered in around lunchtime and a very helpful young man at the desk provided us with our new credenciales (for €1.50 each).
The church mentioned above seems not to be open except at mass times and a notice directs people up the hill.
I almost forgot to pick up my Credential as I was leaving Sarria. Turned around and walked back around the first corner and found a cafe called Matias Italian that was open for desayuno. She had Credentials for 1.50 Euro. Picked it up and got my first stamp too.
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.