I reccomend etsy. You have to watch out for drop shippers, but by being concious you can normally find high quality hand made clothing.
Signs it's a real person making what youre buying:
- Custom fit options (can send your measurements)
- Similar materials used in the different designs
- Photos use mannequins or the same model or have the same background
- The designs cannot be purchased elsewhere
- The blurb gives real information about a real person & their story
Also: once you've found a clothing maker you like, you can often buy directly from them, bypassing etsy taking a cut of the profits.
Heres one I found: https://www.etsy.com/listing/1069069106/dark-red-anorakvintage-trench
In general tho, there are some really unique and cool clothing makers on etsy and I've gotten some of my favorite things from creators who sell using that platform.
Good luck finding the jacket of your dreams :)
Edited for formatting