Viktor Levi and Pano are two wine houses next to each other and run by the same group. They serve wines of their own but if you're an expert, there're many more alternatives on their menu. A classic order is a bottle of wine with cheese plate but they also serve good food if you like.
They're located next to British consulate, but simply at Galatasaray school you'll notice vehicles passing across the Istiklal str., follow them and you'll notice both on your right. Link is here in Turkish but click "resimler" tab for photos.