Spain has well-established national pet store chains — Kiwoko and Tiendanimal are the two largest, under common ownership since 2020, with branches across most mid-sized towns and cities covering food, bedding, toys, carriers and basic grooming supplies. Both operate alongside a significant online ordering business, which is useful if a specific product isn't in stock at the branch nearest you.
Major Spanish supermarket chains — Mercadona, Carrefour, Alcampo and others — all carry a pet food and basic supplies aisle, usually the fastest option if you just need to restock food or litter while already doing a regular grocery run rather than making a special trip to a dedicated pet store.
For anything more specialised — a particular prescription diet, a specific brand your pet is used to from home — it's worth checking availability before you travel rather than assuming a Spanish branch of a familiar-sounding chain stocks exactly the same range it does elsewhere, and Kiwoko or Tiendanimal's own websites are a reasonable way to check stock in advance.