To buy diapers without your parents knowing you can get a prepaid card from Walmart and add money to the prepaid card.
Then you can go online to websites that sell adult diapers and they will ship the package to your door discretely so your parents should not find out about it.
Or you can have the diapers held for pickup at a UPS location and then go pick up the diapers that way and bring them home on a day your parents are not home.
Most if not all incontinence supply websites that sell diapers ship the diapers in a plain brown box with no indication that they are diapers in the box.
You could also go to a store such as Walgreens and take a back pack with you and leave the back pack outside on your bicycle and then go into the store and purchase the diapers and then when you pay for them bring them outside and put them inside your back pack.
Then bring the back pack inside and your parents should not notice.
That's how I got diapers inside the house past my parents when I was a kid living at home.