You can also try the sporting goods department in the usually HUGE el Corte Ingles department stores. There is one in the center of Barcelona, near the Ramblas. There is one in Pamplona, sort of near the bus station.
Along the Camino, you can find them in Burgos, Leon, and Santiago. But by the time you get to Santiago, you usually do not need hiking gear. Their merchandising and inventory reflects this.
Earlier on, further east on the Camino, and at Barcelona, expect to find a fairly comprehensive assortment of gear. It is NOT as extensive as Decathlon, but I usually can find any last minute item there.
I hope this helps.