You can make these yourself actually. I'm using the same concept from my original Dry-Erase Game Tokens post.
1. Procure yourself some dry erase sheets (I use GoWrite! Dry Erase Sheets) and some playing cards. I happened to have some blank Bicycle playing cards, so I used those. Any work though.
2. Cut out a rectangle from the dry-erase sheets that are smaller than the playing card. I used a rounded corner paper punch to make them look nicer. Not necessary, but a nice bonus.
That's pretty much it!
Purhase: Wandering Panda Store