I got all my reel decks at estate sales, flea markets or yard sales.
You can get them cheap - dont think I paid more than 10 bucks for any of my AKAIs (I have 3 M-9s and a X1000SD)
Generally all of these machines require some tinkering, cleaning, lubrication before use. (assuming they just came out of grampas garage)
Motor run capacitor problems seem somewhat common in gear this old - but some still hold speed well.
And remember there are 1/2 and 1/4 track machines - and they aren't compatible. (just in case you have an old box of tapes you want to play)
Once you're in - AudioKarma has a lot of valuable info.
I think we sometimes think people didn't have "good sound" back in this era, but as it turns out - they had sound that was pretty darn good.
Mostly - I got into this because I had boxes of family tapes as well as other boxes I had procured from local old-timers that I wanted to play and transfer...but after a while I got curious about recording.