I'm in Melbourne, so not that much help I guess,

but my brother-in-law just gave me a tip - a plastics company (sell mainly to signwriters etc) in Nth Melbourne called Alternative Plastics sells off-cuts and "scrap" pieces of varying sizes for about $30 (depending on size).
I'm hoping to pick up a couple of pieces for my marquee . . . but haven't got there yet myself. . .
but maybe worth looking through the Perth yellow pages for similar sorts of places?