If you want to buy diapers without your parents knowing you should take a backpack with you to a store such as Dollar General.
Then if you have to just leave your backpack outside and then go in and buy some diapers which would be whatever you want to buy.
However I recommend for a good diaper that will fit almost anyone with ease or at least partially are the Gentle Steps Size 7 diapers.
If you're under 18 then they would likely fit you especially if you have a waist size under 32 inches.
Then just put the diapers in your backpack and then bring them inside your house.
Or you can order diapers online and they will ship discretely and then intercept the package before your parents see it.
Just keep watching the tracking information or have the package held for pick up.
I wish my parents would've never potty trained me.
My parents never made me wear diapers all the time or at all past potty training.
Although when I was 8 years old I had to wear diapers the rest of the school year which was Pampers plastic backed diapers.
It was school policy then that if you wet your pants more than once per week then you had to wear diapers for the rest of the school year.
I had wished my parents made me wear diapers all the time at home.
It's unlikely your parents will make you wear diapers and I would just not tell them and buy the diapers yourself and wear them without them knowing.
You can get some cheap diapers from dollar general which are Gentle Steps Size 7 diapers for $5.00 per pack and they have 20 of them in a pack.
Gentle Steps Size 7 diapers are pretty big and I'm almost 33 years old and can almost fit into the Gentle Steps Size 7 diapers although they are not a perfect fit.
The Gentle Steps Size 7 diapers do fit by stretching the tabs out a bit and I use some packing tape to secure it around me.
Gentle Steps Size 7 diapers also are very absorbent and hold a lot of pee and are really great diapers that swell up nicely.
I love the SAP in the Gentle Steps Size 7 diapers from dollar general and they are a great diaper for the price.
I have a 32 inch waist so if your waist is smaller than 32 inches which mine was when I was 15 then the Gentle Steps size 7 diapers should easily fit you as a 15 year old.
They're cheap and worth a try and if you cannot fit perfectly into them then take the Velcro strip off and cut the Velcro strip in half and then glue one on each side of the diaper which is what I do.
Then they can fit and you can also add some packing tape to it if you want.
They only cost $5.00 per pack for 20 diapers which is cheap.