Hi Crabbet
Have a look at the right hand column of the homepage of http://www.targetbunny.btik.com. Not as comprehensive as I would have liked but the best I have at present.
Best stuff I found was the A3 size desk pads/blotters given away by reps to offices. Just make certain that it's the blotter type since the paper consistency is more porous and punches better. However these are hard to come by in sufficient quantity to be a serious contender, but, if you find any, grab them.
Another answer is to back the sheet with an old cereal box or the plastic version of corrugated cardboard. There will be plenty of the latter around where the estate agents are closing up.
Always ensure that the target is held firmly and that you are shooting at it as near to 90 degrees to the paper as possible.
Last, and this is the one you will probably like least, change to a flat headed wadcutter pellet or a hollow point with a front lip like the Bisley field.
Sorry for the lecture,
Target Bunny