MFA -- Need help restocking for Summer Wear!

Exactly as the title says, I have saved up roouughlly 500 dollars to restock my clothes for the summer/spring. I live near L.A. in California. I'm also ONLY 15 years old so I am trying to save money due to me growing a bunch.

My primary problem is that I have an ectomorph build. I am about 5'4" and only weigh about 90-100 pound. (trying to gain weigh) So, it's very hard to find pants/shits/CLOTHES to fit me right. I need help!