8)
The technical term for the scope you are referring to is called a viewfinder, typically used between directors and cinematographers to compose a shot.
As Lazlo said, you can look through the viewfinder that's available on the camera to accomplish the composition of a shot.
But, when used on sets that employ larger camera systems, a mobile viewfinder is a necessity. In those circumstances, a viewfinder ensures a greater a level of accuracy in the outcome of the shot, because it allows the director and the DOP to collaborate in the pre-visualization process as they strategically scale the next shot.
Mini viewfinders, found on sites like filmtools.com can cost a few hundred dollars. Where as, the viewfinders that you've seen directors and their DOP's meticulously peer through on set for big budget movies are priced considerably more.