Entry requirements section (Visas) - from 29 October 2018, self service e-Visa kiosks and Wi-Fi areas where visitors can apply for an e-Visa on arrival will no longer be available in Turkish airports; you must apply for your e-Visa before you travel
You can find more info on the Turkish government website e-visa application system.
This site is the official site for Turkish visa and the cheapest way to get your visum.