Alekz said Apogee Scintilla, for example. http://www.apogeespeakers.com/scintilla.htmQuite amusing, these.
Fortunately they are ancient, and very limited production, so nothing an amp manufacturer need to worry about. But a fun bit of history.
Yep, their current ones are 2.5Ohm nominal 
Re Apogee speakers: ancient but very much alive! And when rebuilt better than new. 
That said, agreed commercially not a big deal to forego designing an amp to be capable of handling loads of some of the Apogees which get even worse when driving them actively. In short, 1 Ohm driving capability is very much a concern for me. Seems the BHK Mono’s might have a shot though.
Ooh, there’s always Bob Carver’s line sources:
http://www.bobcarvercorp.com/#!als/cwgy
Cheap at $18k . . .