I use largish foam pads 10mm X 25mm X1mm thick. I buy a reel of 1,000 pads and then cut the pads down if I need to. They are really good and I've been buying them from the same source for the last 5 years or so. They seem to last for ages, although I don't often use them for decoupage as generally I prefer a clear gel for that.
Lynn xx