They are called "cable entry grommets" but the one shown would have been factory fitted, they are either shrunk, glued or welded as part of factory assembly.
I doubt you will find a rectangular one but you can proably find round ones, the hole where it leaves your equipment must match of course.
Tip: invest in some cable lubricant, it's a special synthetic oil that evaporates without trace. Putting one drop on the cable before sliding the grommet will make assembly much easier and avoid stretching the insulation.
Brian.