First off why do you want diapers? Are you having an urinary incontinence problem during the day or wetting the bed at night?
If you're wetting your pants in the day or wetting the bed at night you should tell your parents so they can have you see a doctor to be sure nothing serious is going on with your kidneys or bladder.
If you just want diapers you can just order some online and they'll ship discretely to your door and then you can track the package and grab it before your parents see the package or just say the package is something else.