According to Den Murray :

I'm trying to locate (in the US) a *grounded* (i.e.3-prong) plug in switch
tap.

I've got a standard 15amp outlet that I have an appliance that I need to be
switch on-and-off-able (is that a word) at the outlet.

Leviton make a non-grounded version (2-prong in, and 2-prong out), but I
need it to be grounded.

Any ideas?

A cheap power bar with switch.

Yes, you can build something a bit more specific to your purposes,
but probably not for less than the $5 or so you can get a power bar,
nor quite as pretty ;-)
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.