I found the Avery vinyl sample card at:
Avery Vinyl Sample Card
I eventually figured out the purchase process:
1. Enter quantity in the box at the bottom of the page
2. Click Add to Shopping Cart
3. Go to Checkout
I guess it should have been obvious, but it took me a while.
Cost for one sample card was $5.09 plus $7.40 for shipping.
If I had thought a little more I would have ordered a dozen, sold them at a small profit, and saved other people some money. Some enterprising person still could.