Harajuku is the place to go
Recently, I went to Harajuku's Takeshita Dori to do some shopping. This is probably one of the hottest spots for Gyaru, where even some of the store employees have the Gyaru make-up and fashion. As the stores are aimed for young Japanese girls, the prices are also reasonable and many stores often have sales. Although I cannot remember the name, there was a jewelry store that sold all items for 315 yen!
Whether you're looking for cute Japanese clothing or cool tomboy outfits or costumes from mangas, Harajuku is the place to due to its variety of stores, reasonable pricing and fun atmosphere.
Refer to this link for a map of Takeshita dori - http://www.cafe-crepe.co.jp/takeshitamap.html
The street is right across Harajuku station, so do not worry about getting lost!