Istiklal extends all the way from the exit of the funicular tunnel on Galata Hill to Taksim Square. It is a very wide street that's packed with stores and cafe. Many of the stores are American/European chains such as Gap, Nike, Adidas, etc. but there are some good local outlets. Watch for a few of the interesting passageways that run off of the street and ride the small tram which runs up and down the street if your feet get tired.