If you don't physically need the diapers due to incontinence or bed wetting then most likely your parents won't buy or make you wear the diapers.
You could however just tell your parents you want to wear diapers and see what they say. Otherwise I would recommend just going out and buying some diapers with your allowance if you really want them.
Other than that I guess you could fake bed wetting or fake incontinence by wetting your pants or by purposely wetting your bed to see if your parents will then buy you diapers but they might take you to the doctor to have tests done instead of buying you diapers.