Public facilities are common in the tourist areas of major cities and resorts and at archaeological sites and other attractions; in most, a custodian will ask you to pay a fee (typically ranging from 50 kuruş to 1 TL). In many public facilities, including those in some small restaurants, toilets are Turkish style (squatters) and toilet paper is often not provided (to cleanse themselves, Turks use a pitcher of water set next to the toilet). Sometimes it's possible to purchase toilet paper from the custodian, but you are well advised to carry a supply with you as part of your travel gear. Alas, standards of restroom cleanliness tend to be a bit low compared to those in Western Europe and America.
If you're away from tourist areas, look for a mosque, as many have restrooms as part of the complex of washing facilities for Muslims to perform their ablutions before beginning their prayers. Standards of cleanliness at mosque restrooms are usually higher than at public facilities. Most, but not all, restaurants and cafés have restrooms, but, again, the standard of cleanliness is extremely variable. In general, five-star hotels have the best facilities, and the staff rarely raise any objection if restrooms are used by foreigners not staying at the hotel. Many gas stations have restrooms.