In Turkey it is not easy to buy a local SIM card. It is mandatory to register with a passport and a lot of shops insist in instant (and very expensive!) registration of the device you want to use with the card (see Info Box).
Currently tourists are only allowed to take one personal device with you, which you can use for up to 60 days via roaming or a local SIM card.
After this 60 days grace period the device has to be registered (i.e. paying a registry-fee of currently approx. 40 €), to further be able to use it in Turkey.
That's why I am planning to list alternatives to local SIM cards here soon.