No worries Phil.
You need to go into the pictures & picture frame department.
Look for the (large-ish) pictures and frames that are generally stood in big cardboard boxes becaise there are too many to fit on the shelf above.
Look in the bottom of the boxes.
The plastic bits aren't in all of them, some have carboard spacers, some have nothing at all, but pictures and frames that are about 3/4 inch thick tend to have a better chance.
They are easiest to see if the box has had some pictures taken out, and they don't cover the entire bottom of the box, just down the edges so that they act as spacers.
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The shapes would be almost impossible to cut manually, but they are really useful for mass producing fast terrain - we made a dozen 15mm WWII ruined buildings yesterday afternoon, including piles of rubble, texturing the base and painting, if we were manually cutting doors and windows it would have taken far longer (and cost more) although might have looked better, but our Stalingrad project needs another 56 generic ruins and 19 wrecked industrial buildings, so speed is of the essence.
We might have found another source today - will try to confirm tomorrow.