All materials needed for making Mickey Mouse ears can be purchased at a craft or fabric store. If you don’t have any felt, you can paint or color the cardboard circles black or you can even paste some normal black craftsheet on the cardboard. You could glue together several pieces of sturdy construction paper if you don’t have cardboard for the ears. The material you use for the ears should be stiff enough so that the ears don’t flop over after affixed to the headband.
You can also use the bottom of a bowl to create the circles you need for the ears.
You may need to fold the ears up and forward to encourage an upright position.