I love using sheets of craft foam, the cheap stuff you get at Hobby craft and similar.
I drew my design using a pen that would not rub off too badly, and then I cut it out. In this case, because the design was a bit intricate I used a craft knife, but I have created blocks with simpler designs just using scissors. (I have also burnt this stuff with a soldering iron, but you need to work in a very well ventilated area if you are going to do that. It gives off a rather nasty whiff).
Then I stuck it to a bit of card - and voila another print block ready to be pressed into action.
And a page from my sketch book - a bit rushed so the paint was a bit blobby, but I was just using simple acrylics.
You get the picture however.