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.
There's a cathedral http://www.diocesisdesantander.com/catedral-2/ no pilgrim's office. I vaguely remember an hour or two squandered enjoying the building while various, variously enthusiastic, servants spent time trying to track down a sello for me
Your overnight accommodation, one of the Tourismos or the bar you grab breakfast in will all do for a first stamp.
As I remember you have to go to a door outside to get the stamp at the cathedral in Santander. Almost like the staff entrance, maybe the door to the presbytery. The first year I walked I asked and that's where I had to go. The downstairs chapel is worth a look if you have a spare half an hour and the main church is pretty sweet as well.