Before coming here, be sure to have adapters/converters for your chargers and other electrical equipment. Turkey uses a single phase, 220 Volts, 50 Hertz electric system. I also suggest you take a surge protector with you to defend your equipment against sudden voltage changes. Better be safe than sorry.
There are large electronics shops in Turkey, especially in Istanbul, where they are on almost every corner. Outside Istanbul, there are some express shops in the cities or larger ones in shopping malls. The well known brands are Electroworld, Darty (international companies), Bimeks, TeknoSA, PC Gold and Vatan Computer (Turkish Companies), which are open 1000 - 2200 local time (10AM to 10PM, but you should get used to 24 clock while you're at it - Ed). As a traveler, I can advise you to prefer international first, and then Bimeks and TeknoSA. The manufacturers have international warranties, thus if you experience any problems with your hardware when you get back to your country, you should be fine with the warranty.
The prices are the same/a little bit higher compared to the United States, taking currency into account. The Turkish Lira is at an historic low relative to the US dollar, so non-tech goods and services are relatively an excellent deal. There are no mail-in rebates (except those HP tried to make for a couple of years, to no avail.) Before deciding to buy any hardware component, ask the final price, because there is an 18% Value Added Tax on electronics plus 8% Excise Tax. To draw customers, sellers are tempted to write prices excluding taxes and including a cash discount, so you might face ‘a little bit of’ 27.5% increase in the price when you are at the cash register presenting your credit card.
You will be able to find pretty much anything in the global electronics market in Istanbul's shops. The traveler’s urgent needs such as memory cards, USB sticks, portable disks, adapters etc. can be found everywhere and with very agreeable/world standard prices. However, certain brands are not available. For example, if you want to purchase Buffalo RAM for your computer, forget it. Buffalo is not in the Turkish market.
As I have mention in the introduction section, do not expect to find the latest and greatest hardware in Turkey. The manufacturers prefer to launch their products in the American or Asian market first (depending on where they are themselves located) and then to the European/Turkish markets. An example is the Acer Aspire One, 10" model, which launched in the U.S. on 10th February, but is still not available here as of March 16th. Turks are also still waiting for Android phones.
Mac users will be happy to find Apple Stores around Istanbul (the page that the link points to is in Turkish, you can request assistance from your hotel's reception desk.) However they are limited in number, so it will be wise to check the whereabouts of these Apple Shops around your hotel in advance.