If there is an Asian or Korean store near you, you will be able to buy pre-sliced beef at the store. If not, buy beef sirloin, freeze it and then slice very thinly with a sharp knife. After the beef is sliced, cut them into smaller bite size pieces. For authentic Bulgogi, you will need 1/2 cup of pear juice and 1 teaspoon of Sesame Oil to marinate the meat.