Plastic card stock

You can order direct from www.Plastruct.com Alot of online stores carry Evergreen Styrene as well

If you live near a Wally-World, check out the plastic signs they have for Garage Sales, etc.. They may be the right thickness for your needs at a fraction of the cost.

Check your yellow pages for some one who makes signs. For several summers when he was in college, my oldest boy worked for a company which made small signs from several types of materials one of which styrene. The plastic came in 2 ' x 3 ' sizes in .010 thru .040 thickness. He would scrounge scraps and seconds and give them to me. You might find someone who will sell scraps and seconds very reasonably. Or tell you to come and get what you want. Might be worth a try though.

Check the yellow pages under "plastic suppliers" for one in your area. The one near me has a remnant section and quite a selection of styrene, acrylic, etc. as well as glues and other stuff. I've also found great shapes like tubes, domes etc.

I use new car license plate frame plastic inserts. Free.

Also try asking small convienance store operators if they have old signs some of the signs are styrene