The other place mentioned mentioned by TFC i think must be Casanova, they have two stores as well, casanova foto and casanova professional, they are quite close to one another, about a block away. I visited them both for the first time a few weeks ago and was quite impressed, their film is much cheaper and better stocked than arpi, but i dont know about the processing.
arpi is a great place, the floor with all the second hand camera gear makes me drool and the museum is definitely worth a visit but since my last trip over a couple of years ago i noticed that the film and darkroom section is smaller and less well stocked. one refrigeration unit that previously was full of film and instant stock is now given over to inkjet cartridges and another was almost empty. also one fridge full of large ilford paper was actually switched off and noticeably warm!
anyway, I hope you enjoy your stay in bcn, its an amazing city, one i never tire of returning to. while your there if you get the chance be sure to visit the exhibition: "Prague, Paris, Barcelona. Photographic Modernity from 1918 to 1948" at MNAC, i spent hours there wandering around with my brother, time well spent!