Hard by the Isthmus is the Basilopoulos grocery story, which in itself is worth a visit on the way to the ancient site. The first two pictures below show the several story store, which in fact is more than a grocery store, it also has everything from clothes to toys, to tools on its upper floors, but we will focus on the cuisine.
But perhaps your palate prefers the more traditional Greek fare— namely the Kalamata olives and its cousins. There’s something in this grocery store for everyone.