its builders insulation, available from a good buiders merchants....it comes in sheets, if you only need a small amount you may be able to buy an off cut or even a damaged half sheet for a few quid....
i go in to my local place and just ask for 'blue foam', there is blue and pink varieties, i find the blue seems to be a bit tougher and less prone to chunks ,dings and marks than the pink !
its used for insulating cavities in walls and bay windows and such like,
its a dense polystyrene type material and as most polystyrenes is does not like solvents of any kind, it willl glue with solvent free 'no nails' type adhesives, it will also glue with waterproof PVA but it does take a long while to dry !
there is also a white material which is very similar - its specially designed for model making and technical installations and applications, its tougher and slightly lighter, if memory serves me it called ROHRCELL - its alot more expensive than the builders insulation too !