If you are looking for a specific or rare item you can go to Sirkeci area which is very close to Eminonu district.tere are bunch of stores that are selling camera parts,they even do repair.
You can always use online shopping websites too.
The best place for finding photography equipment you should go to Hayyam Pasajı, Sirkeci, Eminönü. Take tramway in Eminönü direction, and get of Sirkeci Train Station. Hayyam Pasajı is just 100m from the Sirkeci Tramway Station. Beside the brand new equipment you can find second hand equipments,too. Most of the shop offers 2 choice for the brand new equipment: 1- Gray market products (You only get european warranty and shop's warranty), 2- Proper brand new product (which has warranty in Turkey). (Because of the high tax rates in Turkey, most people prefers greymarket products).