If any of you wish to tell me that it is in fact easy to find such pieces of cardboard, you needn't bother. I spent thirty pounds on bits of cardboard that day, so I should be set for a good few weeks to come.
There was also smaller bits of cardboard - 1 3/4" x 3". I'm still trying to work out why those dimensions were picked. You'd have thought that a golden rectangle would be the more sensible shape. I bought a load of those as well.