The WI-FI internet has been installed in all hotels and restaurants, cafes in Turkey especially in Istanbul. You must ask for password to waiters or reception to use internet. It is extra cost in some hotels. So it is better to ask before using it.
But we advise to purchase a Turkish Simcard when you land to Airport in Istanbul. Turkcell and Vodafone are the 2 best operators. You pay only once with reasonable price and use it until leaving the country.