I agree with everyone on the clothes stores. DD was just 12 and is 5'6" and weighs between 125 and 130 and also very well developed for her age. She is an adult size, but some stuff is too adult for her. Old Navy is good, as well as we have found some good stuff in Walmart and Target.
I have a similar problem with my DS that's 12. He's too big for most boy sized clothes but even a men's small is too big. We shop old navy alot because they do have smaller sizes for men. Most of their jeans you can order in the waist / length you need instead of a certain size.
I'm just glad I don't have any girls! I dread going through the stores myself - some of those sales clerks