If talking foamboard I use Daler Rowney from my local art shop, the stuff sold by the Hobbyzone group is also said to be good and a little cheaper. Though nothing over here is as cheap as the Dollar stuff available in America and by a long way £4 is probably the cheapest for a A1 sheet which is about 6 dollars.
If talking insulation foam look for Blue or Pink verities but prices have gone through the roof in the last couple of years after more EEC meddling.
Paul