If you're able to order diapers online and have them shipped to you then that would be easiest.
Most diapers that are sold online are shipped to your door discretely with no mention of diapers and you could just say that it's something for your computer or just something you ordered.
Or you can have the packaged dropped off and held for pick up at a UPS location and then pick it up there.
But I would just order the package and then have it delivered to my door and then get the package before your parents see it.
Just watch the tracking often and when it's delivered go out and get it and then bring the package into your house and hide them in your closet.
Or if you go to a store to get some diapers you can place the diapers in a backpack and then bring them home.
You could hide them somewhere near your bedroom window and then open the window and put them inside that way.
Or leave your window unlocked so you can open it from the outside and then put the diapers inside your room that way.
Wearing diapers around your family or even parents without them knowing about the diapers you're wearing is pretty easy.
When wearing a diaper around family or parents just put the diaper on as normal and then put a pair of underwear or boxer shorts over the diaper.
The underwear or boxer shorts will help to cover up the diaper to make it nearly invisible and if your shirt rides up then they will just see the boxers or underwear and not the diaper.
That works for hiding your diapers in public as well.
Then just put a pair of pants or shorts on over the underwear and diaper and then nobody should notice.
I used to put underwear over my diapers before going in public or around family but now I wear just the diaper under my pants or shorts and nobody notices the diaper.
Most people are too busy with their own lives to be staring at your bottom and they are not really looking to see if you wear a diaper.