Have you considered military surplus? The Polish officer's coat I bought two years ago for 60€ is the warmest winter coat I've ever owned, warm even in -30°C.
I know finding these is a bit of a dice roll, but Varusteleka seems to get shipments semiregularly, you can subscribe for an inventory alert, and they sell these on both sides of the Atlantic, I believe.
You can button these to both directions, if that's a criteria, and the backwaist can be adjusted, somewhat.