I set out to prove you wrong and searched www.trade-a-plane.com for 172s
in the 30-40K range, and .... you're (almost) right. I found mostly 196x
with a sprinkling of early 197x, and a couple of late 197x.

I think you might be looking in the wrong places. Those people who
advertise on trade-a-plane are advertising there because they couldn't
sell their plane locally, maybe because their price is too high. Those
aren't sold prices, they are just advertised prices.

I set out to prove your right... ;-)

I went to controller.com and found a dozen 1970's 172's for $40,000 or
less.

http://tinyurl.com/yk8kxf

Visiting airports and looking at the local FBO bulletin boards is a
great way to find airplanes, as you suggested...

Best Regards,
Todd