I'm new posting on this forum, so i hope this post isn't too old, just reading and thought that I'd give you a very specific location, in case you're still looking. You can find lot's and lots of these babies in a town called west alexander, south western PA. I used to live across from a mine there. The mine is now a place called Hanson Engineering and as you enter the parking lot there is a creek that runs under the road. the banks of the creek are covered in river rock and you can find copperheads hanging from the trees, as well as under those rocks.
Hopefully they are still inhabitants of the area, because that was a long time ago. But they were very plentiful.