You can order some adult diapers online through incontinence supply websites and they will ship the adult diapers discretely to your door and no one should be able to tell what is in the package.
You can also have it shipped to a UPS store for pickup for a small fee and then you can just sneak the box into your house.
You can get a prepaid debit card from Walmart that you can use online to purchase the adult diapers or other stuff online.
There's a fee for loading the card and using it though.
I order some of my diapers through healthykin they have cheap adult disposable diapers and ship discretely to my door.
No one can tell what is in the box that I ordered.