I bought it online for $5 bucks. I wish I knew too! It's got a scratchy, rough texture actually, is very lightweight and nearly see-through, and when you look close at the thread it looks like it's got little gaps in it. Not gaps as in holes necessary, but just... eh. Like the thread skips "thickness" but in a regular pattern. It's all very fine though.
I have no technical terms, so sorry!