"The advantage to the swivels is that the ends of the adjusting screws
do
not get faceted. As I understand it, once the tip is faceted, accurate
valve adjustment is very hard to accomplish."

Agreed. Once the tip of the screw starts to flatten out a bit, you do
the obvious..turn it in to reduce the clearance. But that also presents
a new 'unworn' portion of the screw, so your lash adjustment becomes
tedious. The clearance goes away very quickly; the new surface on the
screw is higher than the worn spot, and the screw is moving towards the
valve stem at the same time.

"So the Berg setup -- shims under the towers -- is bogus? "

No. I've used the berg shims. You don't want to use washers because
that can cause the rocker tower to fracture, but using a berg shim
which covers the entire surface is not a problem in my opinion. I don't
like the fact that the berg shims are a press-fit on the studs, Berg
says they need to be like that..I disagree. I usually drill them out
just a tad so they slip on easily. I visually check to make sure they
remain lined up as I tighten down the rocker shaft.

"Back to my question: where does a squirrel find new or really good
condition stock rockers w/ the stock M8 thread?"

No idea. Have you confirmed that berg does not offer the swivel
adjusters in the larger thread? If you haven't run the M8 sized ones
yet maybe they will exchange them for you?

Chris