Where to connect a c-wire on a furnace with existing two wire mercury thermostat

Okay... After reviewing your new photos, I think I have it figured out.

This is what your system looks like

^{You'll have to excuse the glitter pens, it's the only thing I could find on my wife's desk.}

It's pretty simple actually. When a thermostat calls for heat, the valve for that zone opens, and the boiler is also signaled via the auxiliary switch in the valve.

To get a C wire to one or more of the thermostats, you'll simply run a wire from the other side of the transformer on the wall.

^{NOTE: If this diagram is accurate, the white wire to the thermostat is R, while the red wire is W.}

The original wire from the transformer should be connected to the R terminal on the thermostat, while the new C wire should be connected to the C terminal on the new thermostat. This should supply power to the thermostat, and allow it to operate as normal.

The way your thermostat is currently wired, you'll have to swap the white and red wires to get it to work. So white will go to R, and red will go to W.

To make this fit a more common wiring style, I'd probably rewire it a bit. So that the red wires going to the thermostat, connected the transformer and the R terminals. And the white wires connected from the zone valves to the W terminals on the thermostats.

^{Rewired to fit a more common style.}

Then you'll run the blue wire from the white transformer lead, up to the C terminal on the thermostat.

Since in your case, one side of the transformer is grounded. You can simply use a fork or ring terminal, to connect the C wire to the chassis. Though it appears there's already a wire that's attached to ground, and comes right over near the thermostat wiring. I'd just put my C wire in with the other two wires, in that twist-on wire connector near the bottom of the photo.