If it's attached at the sides, it's called a surcoat, I think. if I were you, I'd get some butcher paper or something of the sort and make my own patterns... Use a rectangle with a hole (TABARDLOL) as your base, and make the shape how you want.
But either way, the material needs to be fairly durable, since it's gonna be on the outside of armor. 3nodding
For the tabards I've been commissioned to make, I bought 60" China twill, but that's because it was of the proper durability, color, and it was on sale. biggrin
Linens are nice for them, but way too damn expensive.
Basically, it needs to feel like it can take a beating, and it's also nice for it to be washing machine friendly.
heart