Have a look at this website:

http://www.alacati.8k.com/photo6.html

Alacati near Cesme is the best set up I saw, lots of flat water, and wind turbines at the end of the bay. They run a wind/kitesurfing comp off the beach at Cesme around this time of year. Cesme is near Izmir, which is about 5hrs bus from Istanbul, but the buses are good quality and cheap.

Most tourist stuff will be shutting down around Oct but I think Alacati is year round. It is the autumn storm season by Oct and the wind can change direction quickly with lots of lightning, but ask the guys at Alacati for the best info.

There are heaps of windsurfing shops there, I am sure there will be kiteshops by now, but not sure about rentals.

Gav