Hi Paul,
I get them from Babykins/Kins. Here's a like to their website, for the page with that pant on it.
http://babykins.com/adult-waterproof-covers.html
It's the polyster pant, 10200TP, about half-way down the page. I've tried the vinyl pant but they tear after a while. The polyester pants will last until you want new ones. After months and months of use, at least 10 hours a day, the plastic around the legs, in back, will lose some softness. The pant still works and all, but I replace 'em.
This is a secure solution, and easy, too. I used to wear cloth diapers and bought those from Kins, too. This pant would work over cloth diapers and plastic pants, as well. It provides the little extra that we need. As I said, never, ever, has any little seepage or leak gotten beyond this simple barrier. The peace of mind is great.