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why do you keep calling it lost??!!!!
Possibly there's nothing special about the foam and it can be sourced from a number of places but polystyrene foam is available in a variety of different densities and some may produce better results than others. A specialist will be able to provide the right advice and, you're not going to use polystyrene with polyester resins.
On behalf of davleigh: Sensible advice I'm sure is welcome but is it really necessary to post comments that just show ignorance? You know who you are...
What are you wanting to make/cast using this lost foam Davleigh?