To get diapers and wear them without your parents knowing you can take a backpack to the store on your bicycle and then leave the backpack outside on your bicycle or take it in on your back.
Some stores may not allow the backpack to be brought in because of shop lifting and if so just leave the backpack outside.
Then go to a store such as Dollar General store and go down the baby diaper aisle.
Dollar General sells a brand of diapers called Gentle Steps diapers in size 7 that will easily fit most adults, kids and teens.
The Gentle Steps diapers are $5.00 a pack for 20 diapers which is really cheap.
Then you can put the diapers into your backpack after you pay for them and then take your backpack into the house and hide the diapers in your closet or somewhere your parents do not look.
Here's a link to the Gentle Steps size 7 diapers which are larger than size 6 Luvs, Pampers etc.
https://www.dollargeneral.com/gentle-steps-jumbo-diapers-size-7-20-ct.html