One Shell can be found in Cabin 2 in the drawer. You may have to rifle through some dirty underwear before you find it.
The second one can be found in Cabin A near the bunkbeds. Smash this with the axe, Kitchen Knife or Hammer, or throw it at an enemy to break it open. Inside are three flares and shotgun shell number 2.
Each shot will only damage the killers by 1HP, but it will knock them to the ground giving you an opportunity to escape and hide.