You will probably pay the same for lexan/poly carbonite as a replacement window. The last piece of 1/4" I bought was $200 from Mc Master Carr. Not the cheapest place to get it from, but we had a lousy parts guy at the time. Home Depot or Lowes may have sheets of lexan.
You might see if Regal plastics or Alied plastics would sell you a sheet.
Home Depot or Lowes may have sheets of lexan.