You can get staps "almost" everywhere: albergues, hotels, bars, restaurants, gaz stations, ayuntamiento's, churches... even in the police office (guardia civil).
Just hop in and say "Tienes un sello por un perigrino por favor"? Even if you don't sleep in the hotel/albergue or don't consume in the restaurant/bar, they will give you one. Between St-Jean-PdP and Santiago, there were only 4 places where they refused to give a stamp. But I did get 297 other ones. I had to buy 8 credencials.
During the last 100km, in almost every bar, the stamp is visible and you need to stamp your credencial yourself.
There is in fact no excuse for not finding 2 stamps a day.
Maybe the only valuable excuse is that the stamp will bite you. But there are a lot of stamps that are leasched