If you're looking for a good bulky adult disposable diaper that is just plain white then you should buy the Northshore Megamax adult diapers which are the bulkiest plain white regular adult diaper available.
If you're looking for a babyish adult baby diaper that is thick then either try Rearz adult diapers or Abuniverse Super Dry Kids diapers or any other adult diapers from Abuniverse.
Bambinodiapers.com has good adult disposable diapers that are pretty thick as well.
I wear Northshore Megamax adult diapers when I want a plain white adult diaper but I also use and wear Abuniverse Super Dry Kids diapers which is an adult version of the old style plastic backed Pampers diapers.
Thick diapers are very good at holding in a lot of pee and can last all day as well.