My local Walmart Store stocks and sells Depends Protection With Tabs which are a pretty decent plastic backed disposable diaper for adults.
I buy them myself sometimes if I run out of my other diapers and am waiting on another shipment of disposable diapers from online.
If you have any medical supply stores in your area then the medical supply stores may also stock some plastic backed or poly backed diapers that are for adults.
Most other stores only stock the cloth backed adult diapers and it's hard to find stores other than medical supply stores that stock plastic backed diapers.
You can find plastic backed adult diapers online at many online stores such as Abuninverse or Northshorecare.com
Northshorecare.com sells a good quality highly absorbent adult disposable diaper that is made with plastic backing and it holds a good wetting and is very comfortable.
It's called the NorthShore MegaMax diaper and is poly backed.
You can find the diapers at the link below.