It's hard navigating through Japanese stores for buys and lot of the time it is expensive but there are stores to buy your gyaru stuff easily and cheaply.
Based in Pennsylvania this lady sells gyaru off-brands for inexpensive price. She has relatively good feedback. She sells clothes and magazines for gals. There isn't much complaint about what she sells. to my knowledge if the item is considered cheap she gets rid of it. This is where every gal should start her wardrobe at. Not much use for Gals who already have a huge wardrobe full of name brands.
From my knowledge most of her items fit small or mediums so email her to see if she can stock a bigger size. I doubt most of them will fit bustier gals.
This website is every western gal's best friend. Why? Because this is your best place to buy good. Based in Hong Kong they have everything you would need for your gal wardrobe. But I'm telling you DO NOT BUY ACCESSORIES THERE!. Their accessories are over priced. Better off buyin accessories (and jeans) at your local mall or ebay. They are not cheap at all but you will find good bargains.
And be careful, their stuff tends to be over categorized and unwell categorized but they are the best place to buy clothes.
Gyaru Comm Sales:
For those who want name-brands this is the place to go. The only thing is you must be out rush everyone for a bargain. Not the best way of going about things but it is the best for a cheaper price.