The only place where you can find Pokemon diapers is through China.
China and huggies have released Pokemon diapers under the Huggies diaper brand.
I've never tried them because I cannot get any access to them but they would just likely be baby or toddler sized diapers with Pokemon on them.
I wish I could get my hand on some but you could likely put some Pokemon designs on adult diapers of your own if there is a way.
I wish they were selling those Pokemon Huggies diapers in the USA but as of now I have not seen them.
There's pictures of the Pokemon diapers on the website at the link below.
https://nintendosoup.com/official-pokemon-diapers-out-in-china/