There are many shops in the Kadikoy area that sell dried fruits, nuts and spices. You could also go to Migros or Carrefour. These are better places to buy than the Egyptian spice market in the old city area.
What to buy Walnuts, Apricots, Pine nuts, Pistachios, melon seeds, pumpkin seeds, sunflower seeds, cashewnuts, roasted corn, haselnuts, almonds....saffron, chillies, so much more...
What to pay There is no bargaining in reputed stores, but in the Egyptian market one needs to bargain a bit.
My Istanbul based American friend took me to this, her favorite shop in Misir Carsisi (Spice Market) this summer. They had an array of wonderful spices, herbs, dried eggplant and dried okra strung like leis that were hanging over us. My senses were engaged as I was surrounded by these wonderful aromas and the sight of deeply hued spices. We had tea and water too which were readily offered to us.
What to buy Despite the name which means 'corn', you should buy all kids of spices plus dried okra, pre-mixed garam masala, Turkish saffron (safran), and the item I was hunting down, cocoa butter!
What to pay 3 YTL for a good sized container of Turkish saffron, best prices in all of the bazaar.