Try searching for "great coats", they often have the attached caplet. Honestly, the closest I've seen to this style are Drover's Coats. Here's
one pattern. I know SuitAbility used to sell one, but I didn't see it on their website. Maybe I just missed it. Down Under Saddle Supply
sells pre-made coats, I've not bought clothing from them, but their tack is top-notch, so I'd assume the same for the human supplies. Unfortunately, as their name suggests, they are in AUS, so I don't know that shipping would be any cheaper.