After using all sorts of materials, my favourite is the corrugated plastic used in election signs. The signs are free after elections, last a long time and a lot of shots (minorly self-healing), and take staples very well. If you can find the ones mounted on survey stakes rather than the wire frames, all the better!
I like these too and you can pick up about as many as you want when the elections or events are over.
Spray paint them fluorescent green or pink to more easily see any shots that don't make it onto the target sheet.
