Where is the C wire terminal on my boiler
Welcome Harj!

Looks like your boiler is steam. If it has all of the original equipment, it doesn't have a C connector. Even with new equipment, it probably doesn't have one. If it does, it will be right near where the thermostat wires connect to the boiler. You'll either need to get a plug-in transformer to get power from or you might be able to tap into a 24v transformer at the boiler if there is one. 

This might help you if you want to use a existing transformer:
https://www.doityourself.com/forum/boilers-home-heating-steam-hot-water-systems/514502-need-c-wire-boiler.html