They are not very common. I found some (2 designs) in , I believe. Rocamador in France near the Basilica. Also in St. Jean Pied de Port (4 designs) downstairs in a souvenir shop on right side of main street as you're heading out of town. Also on CP in Valenca. They as noted by others are more common in other parts of Europe. There are several types - one of the more common being an Al shield shaped design. The picture is actually a paper backed sticker that one can carefully remove & sometimes peel the backing off so that it can be stuck to a metal walking stick. Some also in Santiago.
I also collect elongated pennies - one puts a penny and 50 c to one euro in machine and either push button on electric machine or turn crank on manual version and voila! An oval thin medallion with a local design on it. These are thin enough they can be bent around a wooden staff and attached with 2 small nails (brads) through holes you've drilled in them.
Yes, that's a lot of very arcane information and yes, I probably should have other things to do with my life but, well, what can I say.
Good hunting and buen camino!