Unlikely I would have thought unless somebody has just had one reconditioned only to find that they don't need it after all (even more unlikely). As mentioned, they don't like sitting around & if they are sitting around, chances are they are sitting around because they have problems to start with. To suggest otherwise is I think, disingenuous in the extreme. :roll:
I had one sitting around for years, out of my father's old P5B. Eventually I couldn't take Harvey's pester-power any longer & gave it to him to dismantle, purely out of interest. It was toast, even though it had been working when the car was broken up.
As mentioned, I don't think there are many, if any of us hoarding gearboxes in any condition. Perhaps one box which may get used as a replacement for our own car one day & which we would like to keep for ourselves to save finding one in that eventuality. I don't have a spare now however & the one I did have is now just a casing in Anorakland ( :wink: )which needs a total rebuild.
I hope you find one that is a possible 'bolt-in-&-go', but even if you do & it does, there's nothing to say it won't become a 'bolted-in-&-stopped', very quickly.