I recall responding to a similar thread so I was probably the poster in question.
The material is called 'Mac Tac' though that may be a trade name rather than a generic name. It is a clear vinyl like material that comes in rolls. It comes in two types - textured or smooth. You want the smooth.
It is usually used as a shelf covering so it is found in home improvement stores or stores than deal in home hardware, etc.
Excellent for covering documents and maps. The military buys the stuff by the truckload for just that purpose. Used it of years when I was in the military and I still use it today for covering maps.
You can mark on it with grease pencil, erasable marker or even pencil (though I would be careful with the pencil - too hard a graphite and the lead will make impression in the Mac Tac that while it erases will leave a transparent 'scratch' llike appearance on the vinyl).