There is a large choice of supermarkets within a ten-minute drive. The first choice to make is whether you want to shop in Portugal or Spain. It takes around the same time to drive to either location. There are many supermarkets; here are two which give reasonable choice:
Spain - 'Mercadona'
After coming out of Castro Marim Golfe, follow the directions for Castro Marim. When you get back to the motorway, take the turning for Espanha, go onto the motorway for one turning over the Guadiana River, which is the border between Portugal and Spain. Take the first turning off the motorway and follow the road into Ayamonte. Keep on the road until you see a BP Garage. At the roundabout, just before the BP Garage, take a left turn into a small shopping centre,' La Plaza'. 'Mercadona' supermarket is on the ground floor.
Portugal - 'Intermarche'
Take the road into Vila Real past Castro Marim. Just as you arrive in the town, as you have come past the salt marshes, take a left turn, which takes you along the edge of town, past the railway station on your left. At the end of the road, bear right and take the second turn on the right, which is signposted 'Intermarche'. Follow that road until you come to a roundabout and turn right towards the railway station. 'Intermarche' supermarket is on your right.