Olgunla Sokak runs east west between AtatUrk Blv and Kizilrmak Cd. It is just west of the Kocatepe Mosque in Kizilay.
This small street is crowed with people and used book sellers. The books tend to cater to the Ankara University students studying near by. However, if you ask for English books there will be some around, often tucked in behind, and will brought out for you. I have purchased some fairly cheap used classics there.
Try saying: Ingilizce kitaplar var mI?