I think the best option is to get it at the Cathedral of Oporto. It's in the old part of town and overlooks the River Douro. The Cathedral is also the starting point for the walk out of Oporto (we saw the yellow arrows when we were there).
I noticed the post regarding obtaining credencials in Oporto one of the starting points for the Camino Portugues. Does the Cathedral in Oporto issue credencials? I will be travelling in mid Mar.
I got my credential in the cathedral at Oporto in September. I waited until the end of mass and there was a man at a desk at the back of the church who sorted it out for me.
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