Clothes for short/skinny people?

Im 24 years old but im 5'1 and 90 lbs. I have a fast metabolism and the muscle ive been trying to put on in the gym still isnt doing much for me. I was wondering if anyone knew some good places to buy like business casual type clothes or interview clothes for my size? I live in southern california.

Ive been trying on clothes all day. The closest slacks ive found to fitting were in the juniors section, size 0, still too loose and too long. I was swimming in the 00(short) in another store for adults. I have a pencil skirt. Im really picky when it comes to colors and clothes too.