The size of diaper that a bed wetter will need depends on the age of the child or teen or adult wetting the bed.
If the kid is under 10 years old and wets the bed then pampers cruisers size 7 or Dollar General Gentle Steps size 7 diapers can usually fit a 10 year old and under kid who wets the bed.
For older kids that are 10 years and older you may need to buy Youth Disposable diapers for the older kid and for an adult you can buy adult diapers.
There are also Goodnites and other bed wetting disposable underwear but they tend to leak and are not as absorbent as regular tape on diapers.
Diapers for kids are widely available in stores such as Walmart, Target and many other stores.
If the kid is under 10 years old then they can usually fit in a Pampers Cruisers size 7 diaper, Gentle Steps Size 7 Diaper and even Luvs size 6 diaper, Pampers baby dry size 6 diaper etc.
If your kid is a baby or toddler then any of those baby diapers such as pampers, Huggies, Gentle Steps etc will all fit the toddler or baby and even young kid.
If the kid who needs diapers is older than 8 years old then they may need Youth disposable diapers which you can buy online.
However even gentle steps size 7 diapers can easily fit most older kids who are skinny enough as they stretch pretty big.
But Attends and other diaper manufacturers make Youth Disposable diapers when the kid is too big for baby or toddler diapers and too small for the Adult Diapers.
For larger kids and teens they may need to wear adult diapers which are widely available online to purchase.