Trading posts can mean different things to different people and can refer to different types of places or establishments depending on your locale and/or the time period. I'm not sure where you are at Noah, but in the Midwest and Great Lakes region when people say "trading posts" most people are referring to fur trade era sites. That said, some are documented, but many are typically very undocumented and date from the 1690's to 1820's. Find them requires a lot of research and being a good student of history, and some luck doesn't always hurt.