Are any supermarkets open on a Sunday? Travelling to the area from St Malo after an overnight ferry and it seemed a good option .
No major supermarkets (hypermarkets) are open on Sunday.
Smaller supermarkets are open until somewhere around 12:00-13:00 (varies widely) .
Is that the same everywhere along the route?
that's the same everywhere in France.
Look for an Intermarche or Super-u as they are usually open on sunday morning and while a little smaller than a hypermarche they have decent range of products and oftrn have fresh fish counter. We do our weekly shop on sundays at Intermarche but its starting to be a popular time for the French as well! Service stations stock a good range of food if youre desperate!
Just outside Parthenay the Hyper Casino in Pompaire (Rte de Ste Maxient) is open 9 till 12:30 Sunday morning but it doesn't have a great selection of fresh stuff. You'd be better off looking on your route for a Sunday Morning market. There are several Sunday morning markets in France - Neuville-de-Poitou (just outside Poitiers) may be near to your route. As as well as the market with a wide selection of food, two of the supermarkets are open Sunday (there is also a good wine coop near the centre of town)