I have come up with a great carding solution for bettas. When I jarred my first spawn, I simply did not have the time to make cards for them all, and I wanted cards that would be somewhat water-resistant and would last.
I finally hit upon the idea of using CD mailers. These are the perfect size for the jars I have; they are cheap; and they are tough and stand up well to water drips. In addition, you can even use them as a white background for pictures behind your jars.
Here is a link to some CD mailers on Amazon
, if any of you want to give them a try: