Help! Where can i get clothes that will fit me?

I live in south america, in colombia, and no offence to anyone but poeple here are skinny and average, no need for plus sizes. My question is, Im going to the U.S these vacations but im really tiny (ive been like that my whole life its a family thing i guess, i eat lots), and im pretty tall but really skinny. Whenever I try to buy jeans there (u.s)theyre always sooo lose on me and they practically fall out, shirts no problem. I also have a flat chest. Im 15. and no dont tell me to get the clohtes here (in colombia) because this is not what my quetion is reffering to!, is there any place that sells jeans long and skinny?