I use the plasticore that everyone is making signs out of these days. Looks like cardboard but plastic, very light weight, great stuff! You can even use old campaign signs if you cover them with fabric.
Originally Posted by PaperPrincess
I get left over sign material. Some of it is solid plastic, some have 'channels'. Go to your local sign shop and ask if they have any scraps, usually inexpensive, sometimes free!!!