A lot of my stuff is from vinted, with some digging you'll definitely find something. But first maybe try styling your current clothes because if I'm being honest, anything can be gyaru if you have the vibe.
Don't immediately buy brand clothes, I'm begging you! Start small. And one thing to remember is that building a wardrobe takes lots of time. Don't buy too many fast fashion products even though it might seem tempting - yes, have a few in your wardrobe but don't let it consist of only fast fashion.