Hi, @ajloughran asked the question that started this thread back in 2011. However, while he might not be waiting for an answer, someone else might find it useful.Re: getting a Crediential in Granada.
I am literally sitting on the steps of the Episcopal office which is open most of the time (except weekends) and will cheerfully supply you with a Credential for €2.
It’s a few steps from either of the SW facing doors of the Cathedral in Granada.
Location: 37.175872, -3.599558
Google Maps has the opening hours.