The big supermarkets are outside the center of Athens but here are a few mini-markets or convenience stores where you can find useful products. For fruits and vegetables, go to the central market. You can also find some fruit stands on Mitropoleos street off Syntagma Square and on Eolou St, off Omonia Square.

For all your kosher products: Gostijo mini-market

In the Syntagma Area: to be updated. 
In Omonia area: Marinopoulos: 60, Athinas St. 
Bazaar, Aiolou St & Panepestimiou. 
In Monastiraki: to be updated 
In Psiri: Leokoriou St. close to Thission metro Station.
In Plaka: Nikis 41 & Kydathinaion.