The zombies in the suits and the hazmat guys are part of an ongoing series of collectible minifigures. Go to your local ToysRUs and you'll probably see a bin of little packets. What you get is potluck. Or you can buy specific ones off eBay.
I know some people also use the under-the-helmet Darth Vader head, which is gray with scars.
I'm currently gearing up to do something
with all my Pirate Legos and these werewolves I managed to find...