Wherever you may travel, communication remains important to stay in contact with the family, business, friends, etc.
During your visit to Turkey, you might have good roaming plan included within your daily mobile plans, without the need to change your sim card, phone, etc. If not, Turkcell and Vodafone companies provide good service options with reasonable plans. To get a new sim card, you may check the companies' websites and get detailed information.
Note that due to regulations, you need to register your mobile phone with Turkish authorities in order to be able to use in Turkey permanently with a Turkish SIM card. Unregistered phones can be used up to 60 days. As far as we are concerned, there should not be any issue with the registration law with foreign mobile phones used with foreign sim card.