I did it. I bought Guest's Sunn 2x15 cabinet. As I suspected, it was a 200S cabinet, apparently with the original JBL D140 drivers. Not mint, but in pretty good shape. Now I have to get my 200S in top shape. I think I'm getting some microphonics, and it sounds off. I was getting this with another cabinet, though, so it's not the 200S cabinet.
Even my wife says the complete 200S amp looks really good, in an old-school way.