I understand that a lot of store won't let you post the cards :) But what's wrong with carrying them with you and giving one to someone who looks like they may be interested in buying a betta? I do it a lot and it helps :)
I almost got kicked out of walmart because I kept putting up proper care sheets by the bettas, lol. But after a long talk with the manager I got them to put up their own permanent care sheet by the bettas, with correct information on it. I almost cried the day I walked in there and saw that care sheet, I felt wonderful that I had actually done something to help.