YesStyle sells a lot of gyaru-appropriate Asian clothing:
https://www.yesstyle.com
They're based in Hong Kong, but you can order and communicate with their customer service team in English. I've ordered from them before and been very pleased with their service. Their prices and shipping costs are also pretty reasonable.
Otherwise, unless you want gyaru brand items, Western stores like Forever 21, Wet Seal, Charlotte Russe, Papaya, etc. have good stuff, too. You just have to know what you're looking for.