you can get any kind off the shelf at Interstate Plastics:
22435 68 th. Ave. South
Kent, WA 98032
(253) 395-4885
Fax (253) 395-0535
1-800-742-3444
I was just there a couple of days ago, and they have a sweet scrap piece section. I got another nice 3" piece for $5. I love the attitude/service there and just picked up my 1" stock for my next project.