You're how old?
Good lord I don't think I was even allowed to choose my own clothes at twelve. It was so long ago I can't really remember. Well on a practical note, (if this is even a choice) try and keep it cheap because obviously you're probably going to outgrow them fairly quickly I would imagine. I have seen kids outfits, but I think a lot of the time they were probably made especially for them and many items labelled Steampunk tend to be a tad overpriced.
I don't know what style you have in mind, smaller sized Newsboy caps, sometimes called Baker Boy can be found fairly easily (they're still often marketed for women, don't worry about that they're exactly the same) Kids waistcoats are a bit of a specialist thing, they tend to turn up in wedding type shops, but again hideously pricey if brought new. So something second hand perhaps if you can source it, they sometimes turn up on Ebay. Or if you know someone that can sew, or fancy learning yourself maybe.
Shirts shouldn't be too much of a problem, and neither should some styles of trousers again depending on what you are going for. The newsie style is one that's quite popular with the youngsters and obviously it works well with people around your age.