Here is one that Petsmart has online. It also comes with "doggie litter" but some peole just use pee pads. It has 3 sides, and a nice opening for them tho get into. I think it might be worth a try.......... or go to your nearest Walmart and see if they have a cat tray that will hold a pee pad!! (as you can see, it looks like a cat tray... and I don't see why it wouldn't work)
Sorry, I looked, and couldn't find the thread on this subject... Good Luck! Hope you find something that works for ya!