Depending on how the adult is built and how much they weigh as well as their waist size some adults can fit comfortably into real baby diapers.
I have a 32 inch waist and am pretty skinny otherwise so I can easily fit into size 7 Pampers cruisers diapers although I have to stretch the tabs a little.
But I've never had the tabs from a Pampers Cruisers come lose on me and I'm comfortable in them and they hold up to my wetting pretty good and I'm a heavy wetter.
Pampers is also coming out soon with a size 8 Pampers diaper so you may be able to fit in those as well.
Some other countries even make bigger baby diapers that some adults can fit into although you may not be able to get them in your country.
If you can't fit into real baby diapers then you can check out undercarewear or abuniverse for good adult sized baby themed diapers.
The undercarewear diapers are not really babyish but they're cute anyway with the race car print on them and abuniverse makes diapers that look babyish and are really absorbent too.