There is no particular style to indie girls. The basic outline of an 'indie girl' is someone that does what they want. They listen to music that you've probably never heard of, wear what they want and are usually in touch with subjects that allow them to express themselves such as textiles,art,media,music and photography.
There is a typical stereotype of an Indie Girl which consists of messy, bleached, short hair. They wear vintage dresses and wear red lipstick. However there is no real 'trend' they are each different and unique.
You can't go shopping to try and change your image to be an Indie Girl, it's about being yourself all the time, but in a kind of rock chick way.