Union halls are located in most systems, if not all. Specific union halls listed here have shown up in the books.
Breakall Union Hall
From context, the Breakall Union Hall is located on Breakall Orbital.
Neris Union Hall
The Union Hall on Neris occupied a refurbished hangar at the edge of the shuttle port. A lady named Ms. O'Rourke works there and introduced Ishmael Wang to the world of trade ships by putting in a good word about him with the SC Lois McKendrick.
The Assistant Hall Manager was a mousy man named Fredericks. He didn't talk much.