You'll need some dead mice.
These came frozen from the East Bay Vivarium
. Most pet stores will happily sell you pet or feeder mice, but they may not exhibit such nice color variation. They may also be alive; if so, you'll need to be able to fix that yourself or find another supplier. Choose wisely.
Mice can be stored in ziplock baggies in your freezer until you need them. Don't repeatedly freeze/thaw your mouse or leave it in the fridge for an extended period as this decreases the skin/hair quality. An hour or so at room temperature is usually sufficient to thaw your mouse.