Looking for one offcut of composite panel

Hi. Several people here have built enclosed trailers using rigid sandwich panel. I can't seem to find any sources for a smallish piece.
I'm looking for about 450 x 600 (or a little bigger, up to 600 x 750) in either 20 or 25mm thickness. I don't mind if the sides are aliminium of a hard plastic, nor if the core is foam or honeycomb material - it just has to light and rigid.....
Any idea where I might find something like that? What common industry do I look in or phone up?
Cheers.


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