As I recall, paper weight refers to the weight of 500 sheets of the paper in question. So if you buy a pad of smooth finish, 11"X14" Strathmore bristol that says it is 100lb weight, that means that 500 sheets of that same 11"X14" paper would equal the weight of 100 lbs.
Ply simply refers to the thickness of the board. 3-ply board is thicker than 2-ply board.
Some paper companies don't list the weights or ply of a pad or ream of paper, and others do. I can't say why this is for sure. Sometimes with Strathmore products you'll see something written on a pad that will say either 300, 400, or 500 series. It was once explained to me at an artstore. Apparently this is Strathsmore's way of gauging the quality of their paper. A 300 series is the cheapest series, and is considered a good paper for beginners. A 400 series is more expensive, but it has a harder surface that provides more textures. A 500 series would be the most expensive series, and is considered Strathmore's "top of the line" paper. This series contains cotton fibers and is designed to last . It ages particularly well.
I think I've given you accurate information here, though I admit that I'm no expert on paper. I usually like to use 2 or 3 ply bristol board that has a little bit of tooth to it. I try to find acid free board if it is at all possible. In addition to the bristol, I usually use bond copy paper for roughs and thumbnails. I rarely use other types of paper.;pencil;
You might try looking here to find the paper you desire: