Ebay is actually a very good source of them, you just have to get them from reputable sellers. If you have a large camera story near where you live you might look there.
They are also in short supply because I am hoarding them all! . I have 2 FTN's, an F2A, F3HP, F4E, F5, FE2 and Nikkormat FT2 along with 31 non-AI/AI/AIS Nikkors from 8mm f/2.8 to 600mm f/4. I also have a Hasselblad 500CM and 5 CZ C or CF T* lenses from 40mm f/4 to 250mm f/5.6. And they ALL still get used, alongside my D500, D750 and D850. In fact, all of my Nikkors are all manual focus, I don't own an AF lens and probably never will.
Nikon and NIkkors on ebay tend to be priced higher than any other system.
Locally here in the Philippines, they fetch a high price. Rarely do you see it selling under $100.
There is a good reason for it, they are of very high quality, though not always in the best of shape. I am a part time working professional now that I am retired and all I use are manual focus non-AI/AI/AIS for film and digital. They are built to the very highest of mechanical and optical quality. Some of my Nikkors date from the 60's though most are from the 80's and a few from the 90's. How many of today's plastic, electronic Nikkors will still be around and actually in use 50 years from now?