I can source full sheets of styrene...4'x8', 3'x6' or 5'x11' plus a few other sizes...sizes also depends on thickness. The sheets I still have on hand were expensive 10 years ago...I hate to venture a guess what they'd run now.
Get to be good friends with the various retailers in you area...many of the point of sale displays include styrene for the structure and/or signage. When they replace old for new, then you can probably pick them up for nothing. I have some old signage from a light display at the hardware store I use to manage... .8 inch in 2'x4' sheets...who cares if it has printing on one side, that wipes off with a little lacquer thinner.
Check the various signs for sale too, many are good quality styrene...FOR SALE, HOME FOR RENT, DO NOT FEED THE TROLLS....lol Usually they are under a couple of bucks.