I've got to agree with the other posters here. Shadow boxing with weights won't necessarliy help your speed (though it will help you keep a solid technique when you're tired ..... assuming that you punching technique is right? Be careful, because repetitive technique work when your technique is poor will only make you worse.

I would definitely go for the elastic band solution, or even shadow boxing in a swimming pool if you can.

You dont want to overdo weight work at 14 because you are srtill growing and it might lead to joint damage in later life.

First thing ....... get a proper and good trainer!!!