I think the best (and cheapest AND easiest) option here is to hit up every goodwill, salvation army, and thrift store in your area. That's where I found mine.
It's actually a fairly common cut and colour as far as trenchcoats go, I think the most you'd have to do to alter one is colour the buttons.
Hope that helps! They're everywhere, you just gotta find one in your size.
Endless love for Cas. <33