If you can't find thinly sliced beef at your grocery store or Asian market, you can cut it by hand. Ribeye, New York strip, flank, and skirt steaks are all good options. To make slicing easier, freeze the beef for about 30 minutes to an hour. This will firm up the beef and help you cut the beef into more even slices. Once the beef is chilled and has hardened, sliced the beef against the grain, into 1/6 to 1/4 inch. Try to keep it as thin as possible. And because the beef is thicker than the thinly sliced beef, make sure to let it marinate for a least 1 hour or overnight for best flavor and tenderness.