I personally use human pads. It is exactly twice the size of the pad holder, so I cut it and get 2 for one. It ended up more than twice cheaper than wee wee pads, which can be hard to find over here
I have found the thick ones for not much more money

When trying out different brands, I would "feel" the pads in the bag. You can easily tell if it's thick enough or not.
If it was an option, I would definitely go for washable pads

But it's not, and nobody sells them here, in fact no one really uses pads for their dogs. I was wondering if I can find a way to make it..