Corrugated Plastic Signs, Coroplast Full Color Signs Printed on 4 mm coroplast, 10mm coroplast, 6mm coroplast, Discount Signs.
I just called them and they said a single raw stock 4mm 4'x8' sheet in white would run you $24 plus tax. Talked to Maureen...give her a call.
Have you tried the yer local city recycling center fer any leftover coroplast signs from the election?
Something else you might want to try (if coroplast is so hard to git up there) is go to yer local appliance store and see if they will give you some large cardboard boxes that you can use the cardboard from. I know it's not waterproof...but it will git you some material to work with. Once you create some workable patterns...hopefully you will have some coroplast to work with.