It's always daunting to me, even now. The 2 things I hate most about it are the clay (it's so stiff and requires a ton of kneading), and the waiting time for all of the curing (silicone, mold release, resin, and paint).
I get large scraps from work (sign manufacturer). Our supplier is Reece Supply, and their supplier for expanded PVC sheets is a brand called Komatex. The sheets are 4 feet by 8 feet. They come in different thicknesses. The thicknesses I have personally worked with are 1/4 inch, and 1/8 inch.
EDIT: I'm seeing it available in smaller sheets on Ebay and Amazon, so you might try those sources.
Trying to earn a little. I had made some larger claws which were popular, but not convenient for daily carry, so I figured people might appreciate these smaller, keychain-friendly ones.