Go to your local sign shop and ask for coroplast. You should be able to buy a 4x8 sheet for less than that.
Plan ahead next election day and help "clean up" the signs the day after
Seems like the furniture stores around here are starting to flood the major intersections with coroplast signs for their yearly "going out of business" sales. Keep your eyes open, I'm sure you can pick up some signs.
BTW, you can mention beesource here