I get all my plastic from the nice, reasonable people at
www.interstateplastics.com
Or go to your local plastic place.
Ask for a chunk of ABS, 1 foot square (say: I'ts gotta be within an eighth-inch).
Pay full price, no squawking. Be chatty, be NICE.
RIGHT before you leave, ask: "hey, you got any scraps I can play with? Nothing special, I just don't wanna screw the GOOD piece up."
They will LOAD YOU DOWN with free material. Smile, keep looking like you don't have a clue what it's worth, even tho you now know.