well, i recommend not using polystrene. thats pretty much styorfoam or insulation in houses which you can get 2x8 sheets of i think at home depot for about 12 bucks.
but styrene aka hips is what you want and would be easiest to use for vacuum forming
i live near a tap plastics that sells a junk ton of everything. depending on how big youre building a table you can use the for sale signs at home depot.
if you have a "fast signs" or other sign making business nearby they have cut offs and may even sell plastic
hope this helps!