Looking for a Wall Wart Power Supply with an Earth Ground Connection on the Output
The only places i've seen wall wart supplies with an earth ground terminal are on supplies built for the alarm system industry and normally only on the 16.5/24V AC supplies.  

Here is an example

As to why you don't see them.  The ground terminal on mains power is intended to be used to earth ground the enclosures of devices operating at mains, or high voltage.  For example a blender with a 120V motor and a metal case.  This prevents the chassis from ever being at dangerous voltages due to wiring faults or shorts within the product.

DC supplies normally have a floating ground that can be at a much different potential level than earth ground.  There normally isn't a use for a earth ground in such systems.  You'd have to be very careful with it to not cause a ground loop and grounding the chassis to the floating ground is normally done but more for emissions issues as unsafe chassis voltages aren't a concern in low voltage DC systems.