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.
The Mimban stormtroopers' capes appear to be identical to the ones worn by mudtroopers, who's capes are old soviet plash-palatka raincloaks dyed black.
This is supported by the 501st CRL for Mimban troopers: