This was mentioned by someone else earlier in the message boards, but is such a good tip that it had to be reposted.
Bunnings (and no doubt other large masonry etc shops) put corflute on their pallets under tiles, pavers etc. These then are stacked around the nursery/paving areas of the stores until thrown out.
Our local bunnings was more than happy for us to help ourselves to all we could carry.... for free!
While it is not as thick as the sign grade stuff, it is easy to shape, light and just as strong etc as thicker corflute.
For the decorators amongst you, we only found black, but Im sure if you want to search different shops you could come up with some colours
Anyho, just thought you might like to know
Pica & Pippin (Bort and Annas owners)