You can find all kind of shops in Arles, or St Gilles, with the nearest petrol station on the road to St Gilles, just as you enter the town.
For supermarket shops (Intermarche, Lidl, Aldi), it is easier to go to the industrial estate just one exit after Arles where there is lots of parking. You will also find banks, pharmacies, petrol station, bakery etc here
St Gilles also has a large Intermarche and Lidl, including smaller shops on the high street.