Alex Coleman wrote:
I want to mount a mirror on the side of my desktop PC so I can see
through the window to my side without moving my head.

A car overtaking mirror seems ideal and I want to mount it such that it
can swivel and be adjusted. A ball and socket would be nice but I don't
see many of these available.

(A spiral wound metal arm has too much "spring" and springs back a
distance to where it was moved from.)

Where can I get a ball & socket which has a decently wide range of
action?

There are (or used to be) auxillary mirrors at Halfords et al intended
for mounting temporarily on the passenger side, for the use of drivers
accompanying learner drivers (my dad used one when I was learning) - had
a ball/socket mounted on a sucker.

Alternatively, you can get mirror especially for mounting on PC monitors
as you describe (for office workers who don't like being sneaked up upon!)

David