I found these brand new animal cards at the thrift store for like 25 cents or something. They are the Mommy animals and the Baby animals. I made 4 folders of these because there were so many cards. The Mommy cards are taped into the folder, and so you match the baby to the mommy.
You will see throughout all the folder games I've made so far that most are using flashcards or card games that I already had. It is much easier to put them in a folder game than to have them strewn all about the house. Plus it makes the creation of a folder game a cinch!