Look for a local sign shop or auto graphics shop. You may get someone to flip you some scraps for free. I work in an auto acc. shop that, among other things, does vinyl graphics and if someone came in looking for those sizes, I'd be happy to hand some over. It's good for business since you may come back later and spend real money.
If you can't find anyone local, let me know. I'll put some in the mail for the cost of sending it.