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Skips usually

Any trade DIY store should stock it like Jewsons etc.

Failing that, you could just order some online and get it delivered, google is your friend here

There was the bit in the middle as well so I am still being helpful though the OP stated that he didn't have time for it.

The problem with this is that hobby places do give stupid markup because people only want a small amount of insulation which would involve the Hobby store having to pay a higher markup themselves for someone to cut the insulation board smaller for them or for the hobby store to take the time to cut the insulation board down.

If you order from the trade, you're almost guaranteed to end up with a full size board (Which if you put two together and carve it can make for a lightweight gaming table) which you'll have to cut down yourself.

If colour doesn't matter these might be your best bet

https://albadecor.com/en/ceiling-tiles/93-ceiling-tile-t101-polystyrene.html?gmc_currency=2&gclid=EAIaIQobChMIgbihg6bt1wIVhpPtCh02rASDEAQYBCABEgJYbvD_BwE

although....on a second look...I did find this and they have a 6mm version as well for less

https://www.amazon.co.uk/dp/B009A826Y6/ref=sspa_dk_detail_0?psc=1

Don't talk to me about insulation panels, I saw way too many of them in my last job from many different manufacturers.