Electric guitars: What is the piece where the strings lay going to the fretboard?

Coming from the tuning pegs, their is a resting piece that has a groove for all 6 strings to lay on, on their way to the fretboard. If you go to the pickups, you've went way to far. On cheaper guitars, it is a cheap piece of usually white plastic. How can I get a left handed one to put on a right handed guitar?