France has well-established national pet store chains — Maxi Zoo and Jardiland among the largest — with branches in most mid-sized towns and cities, covering food, bedding, toys and basic grooming supplies. Jardiland, which started as a garden-centre chain, often combines a pet section with its main gardening business, so look for it alongside garden centres as well as standalone retail parks.
Major supermarket chains — Carrefour, Leclerc, Intermarché and others — all carry a pet food and basic supplies aisle, which is 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.
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 French branch of a familiar-sounding chain stocks exactly the same range as it does elsewhere.