In the worst case, the Hotel El Suizo in Ferrol, where I stayed has credencial's / pilgrim passports for sale in the glass cabinet just past reception. I seem to remember that the tourist office in the square near the station also had 'proper' passports, as well, as opposed to a single sheet of paper.
But
@Lizanne801 has the best idea. If you are not going tomorrow, or next week, order credencials off Ivar on this website. Then you have one thing you do not need to worry about when you reach Spain.
Strictly, if you read the Pilgrim's Office website (see link
https://oficinadelperegrino.com/en/pilgrimage/the-compostela ), it says that you should obtain a stamp where you stopped and started each day, and at least 2 stamps per day. Therefore, if you reach Pontedeume at the end of day 1, the advice is to obtain a stamp at Pontedeume (the alberge or pension you stay at), and at the start of day 2, you obtain another stamp (this can be from the same place as the stamp as the night before, but with a different dat)e. Any stamps that you can pick up along the way, during the day, at town halls / concellos, churches, bars and restaurants, all help to prove you have walked between the point that you started the day, and the point you ended the day. I managed to pick up 3 or 4 stamps per day, apart from the last day from Sigueiro to Santiago.
Therefore, as
@falcon269 rightly says, let's say you had a rest day in Betanzos. You obtain a Betanzos stamp when reaching Betanzos on the evening that you arrive, and you obtain another Betanzos stamp on the morning that you leave. All stamps are dated. There is no need for any stamp on your rest day, but there is also no harm in obtaining a Betanzos stamp for your rest day either.