The most famous U.S. legal stationery item is the yellow pad. I think the traditional size is foolscap (is that what they call legal size?), but Staples had them even in mini sizes. The yellow paper with blue lines (and red margin?) is so distinctive that For Counsel sell ‘legal briefs’ in the form of yellow-lined underwear.
It was difficult to find a picture of a legal pad, but here is one.