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.