There are plenty of speakers, new and vintage that can be aquired singly, that is not a problem.
The issue is deriving a genuine, single mono channel, not two stereo channels with a centre image which is not the same thing. It may sound picky, but if you are serious about this, stereo with a centre image will not do.
Mono power amps are easy enough, but a mono pre-amp is difficult, have not seen one in ages. I would be tempted to look in the pro shops for such a thing, I doubt they even exist in the hi-fi world these days.
You might be able to find something vintage though, but as always, this will be pot luck.