Finding a Nak - and finding a decent Nak you can use - are two separate issues.
Naks are a bit like classic cars. You might find one for $5,000, spend $20,000 restoring it, but it'll only be worth $12,00 after the work. Unless the project is important, understand that negative equity is part of decent cassette deck ownership.
I don't think there's much middle-ground here. You either buy one that works and put aside several times what you paid for it to get one of the handful of quality cassette deck Techs to get it into factory 'spec, or you find one that's been serviced / has paperwork and go through it with fine tooth-comb.
The number alludes me at the moment, but I once had a Revox / Studer deck that sounded sensational - slightly better than my Denon. Denons are best value - they're superb for the money. I once did an A/B with a friend and he couldn't tell reliably whether we were listening off the play head or preview.
The 1000ZXL Ltd Edition is king of Naks, but rare as hen's teeth and very expensive (an unbelievable piece of kit - mind blowing).
I'll agree that cassette won't flap your trousers like 499 1/2" on a decent A80, nor will it have the S/N ratio, but in terms of mid and top clarity, you'll find a well-serviced Nak or Revox will put up a serious fight.
I have nothing against cassette decks. I think they get maligned because people don't appreciate the maintenance requirements.
Be warned that spending a little extra on a Nak or Revox *could* equate to a lot less in servicing needs. Servicing costs on a good cassette deck can be frightening - I've been there.
Justin