The retailers of "American trad" are usually associated with the East Coast, specifically New England. These are shops like J. Press and the Andover Shop and, in the South, shops like Ben Silver and Eljos.
I've found some excellent, unusual items from trad retailer O'Connell's, which is based in Buffalo, NY. Be sure to check their Customer Pictures section, where the store picks out selected items from their sizeable inventory for display. They have some amazing new old stock Invertere storm coats (one vicuna lined, another alpaca-lined), which are unfortunately not in my size. Invertere Coatwrights (est. 1904) is a venerable English make that is apparently no longer in business. Creator of the "buffer coat", the company seems to have closed down and then attempted to relaunch (link to website placeholder and contact info).