First let me say I’m not an expert on this. But the image you shared has this detail which made me think: “is that a tent?”
In any event it’s more like a rain poncho, very military surplus looking, as opposed to a more winter-weather “parka,” in my opinion.
The 501st has this page which isn’t terribly specific but does have a clue: Costuming:Mimban Trooper - Databank
The key is the term “plash-palatka.” Soviet cape-tent (so I was half right!
.) More details here: Plash-palatka
No clue as far as what specific type you want- if it’s like other Soviet/Russian military surplus I am confident there are tons of Polish, Czech, and many other varieties of this. Just depends on if you need ultimate accuracy or just something that looks the part.