Help me to find C wire
You might be in luck, but it depends on how the Honeywell thermostat works.  I know Ecobee3 lets you power the thermostat using the Rc and C terminals, but I'm not sure if Honeywell does the same. However, this solution might still work, even if the Honeywell is not the same as an EcoBee3.

If you look at the documentation for the boiler, you'll see this.


  


Which shows that the boiler exposes additional wiring, that's intended specifically for powering a thermostat.

You can see these two wires in the photo of your boiler.


  


If the Honeywell can be powered using Rc and C, then here's what you'll have to do.


Install an 18 AWG (at least) 2-wire (at least) cable between the boiler and the thermostat(s).
Connect one of the wires to the wire labeled R in the boiler (as highlighted above).
Connect the other wire to the wire labeled C in the boiler (as highlighted above).
At the thermostat, remove the jumper between R and Rc.
Connect the wire that is connected to the R wire in the boiler, to the Rc terminal on the thermostat base.
Connect the wire that is connected to the C wire in the boiler, to the C terminal on the thermostat base.
Connect the other thermostat wires as they were on the old thermostat.




If the Honeywell thermostat cannot be powered using Rc and C, then you may still be able to get it working.  Follow the same procedure as above, but do not connect the wire between the R wire in the boiler and the Rc terminal at the thermostat. Also, leave the jumper in place between R and Rc.  This may or may not work, depending on how the system is currently wired.