You can make your own piggy bank or buy a plastic one from the store. You can also use another type of animals like a penguin or duck (anything works). It take a little bit of electronic knowledge to make but is pretty simple.
Step 1: Building the piggy bank
Next you put plaster clothes dipped in warm water over the frame and let each layer dry for about 10 minutes.
Once you have a sufficient amount you sand down the layers to make it as smooth as possible (note: it will never be totally smooth unless you take a really long time and prep the wire frame better).
Let it sit overnight to allow it to totally dry.