Do you mean this one?

Looks like a wool, double-breasted six button trenchcoat, but knee length. I can't help with the exact item, but if you google something like "wool trenchcoat knee length for men", you could find something that matches the style and cut pretty closely (the colour would probably be the hardest part). That search pulls up a lot of what are more like jackets, they come to the hip, not a knee length one you'd like, but you should find what you're after. A bit like this, but in green obviously (and ditch the belt! - also the collar on this one is a bit full on):

Depending on your height, you might want to consider more of a "mid-thigh" coat or a pea coat, shorter than Windmark's but longer than a regular jacket. Sort of like this:

Although on that model I reckon it looks a bit too short, you'd want at least another inch for it to be actually "mid-thigh".
Oh God my love of fashion is showing...
Not much help I know (I'm sure Google was the first thing you tried) but just chiming in, good luck!