Buy Gyaru Clothes in Tokyo
If Shibuya is a little bit too hectic for you, then I would personally go to Harajuku. This train station is only one stop before Shibuya. The shops in Harajuku are spread over a wider are, as there are no department malls. Instead you will find quaint little shops tucked away in alleyways. You will find a lot of boutique gyaru shops just by doing a little bit of discovering. I recommend that you start off by heading to Takeshita Dori which is accessed directly in front of the JR in Harajuku station.
There are other places where you can also get gyaru fashion however I found that Harajuku and Shibuya had more than enough shops that it took me two days to see everything that was there. If you would like to find counter fashion that is more appropriate for an older adult then I would suggest you check out one of the peak department malls such as Marui.
If you are in Osaka the easiest place to find gyaru inspired fashion is a Amerika-Mura and Shinsaibashi. The department mall called OPA is the closest thing to Shibuya 190 in the west of Japan. When I was in Osaka a couple of years ago, I noted that the gyaru fashion there was a little bit more laid-back and mature. If you want something that screams out a little bit more personality than Tokyo will have a boutique shop to fit your needs better. If you are into hime gyaru or Lolita fashion Harajuku has a better concentration of boutique shops.