Some things you can do to convince your parents to let you wear diapers is try wetting the bed at night or start wetting your pants in the day.
Some parents will want you to wear diapers for bed wetting or pants wetting in the day but it does not always happen that way.
I would personally not even try to get my parents to let me wear diapers as some parents may not allow it or want you too and they may yell at you for wanting to wear diapers like mine did.
I would just go buy the diapers myself and then sneak them into the house by use of a backpack or something similar.
I used to leave my bedroom window unlocked and then go to the store and buy baby diapers and then bring them home and go to the bedroom window and open the bedroom window and put the diapers in my room through the bedroom window.
I always got the diapers in the bedroom that way without my parents knowing about it.
If you have a 32 inch or smaller waist size then I would say that you can usually squeeze into and partially fit comfortably into Luvs size 6 diapers which are cheap and can be bought at Walmart, Dollar General etc.
Also Dollar General has a size 7 Gentle Steps diapers for around $5.00 a pack for 20 diapers which are a bit larger than Luvs size 6.
I would just put them in a back pack and bring the diapers home that way.
But if you want to order adult diapers online most places ship the diapers discretely then just get the diapers before your parents find out.
Or have the diapers shipped to a UPS or Amazon Locker where you can pick the diapers up.
You can use a prepaid debit card to order diapers online.