how do i get a peashooter?

I've got a disk copy of the supernatural expansion and I entered the code to get the plants vs. zombies content which came with the tshirts ect.

The DVD case mentions using a peashooter to protect plants during full moon from zombie attacks, just wondering how I might get one since I can't find it to download on store content and it's not already in the game.

Any help with this would be great thanks =]