What should I search for on the google maps for such a store; I already tried "kahve çekirdekleri" and "kahve" but the results seem to be mere coffee shops.
I'm in the Beylikduzu zone in Istanbul.
I believe you are looking for "Kurukahveci" or "Kurukahvecisi", which is "seller of coffee beans or ground coffee" or "coffee store".
The most popular "Kurukahveci" is Kurukahveci Mehmet Efendi, which is pretty far (approx. 35 km) from the Beylikduzu zone in Istanbul, for coffee beans. It's just outside the spice market.
I think what you're looking for is Çekilmemiş Kahve. You should be able to find some in your local Migros. They have different types of Çekilmemiş Kahve, and the prices will be relatively cheaper than if you were to go to the Grand Bazaar.
If you're solely after Turkish coffee beans, you could try for çekilmemiş turk kahvesi from places like Kahve Dunyasi.