The diapers that fit a 12 year old would be Attends Youth diapers or Size 8 Boundless diapers from Cuties.
Other size 8 diapers should also fit a 12 year old.
Size 7 diapers can also fit some 12 year olds depending on their weight and waist size.
You can easily find size 7 diapers in most stores including Walmart, Dollar General, Dillon's etc.
Or even online you can buy size 7 and size 8 diapers and even Youth Diapers.
Youth diapers would surely fit a 12 year old and they come in pull up and tab style diapers.
These Youth diapers are made to fit older kids that are too big for toddler size diapers and too small to fit into adult diapers.
However the size 7 and size 8 diapers are made better than the Youth and adult diapers and are more absorbent so if they can fit into the size 7 diapers or size 8 diapers then that would be a better choice.
My 10 year old nephew who stays with me sometimes wears a size 7 Gentle Steps diaper from Dollar General as they fit him just fine and are very absorbent.
He only wears them at night as he wets the bed and they keep him and his bed dry.
I'm sure he could still fit in them at 12 as he's skinny enough.