A genuine U.S. Navy peacoat, preferably from the 1970s or earlier.

I can't post a direct link per subreddit rules, but there are plenty of U.S. Navy-issue peacoats on Ebay. This article talks about how to identify them so you can make sure you're getting the real thing.

32-oz Melton wool, very warm, durable, timelessly stylish, and affordable. You can pick one up for under 100€. I have one from the early 1960s and it's probably the best-made coat I own.