Look at www.taco-hvac.com for a zone controller. This will have a low-voltage control module that a standard thermostat can control and a relay to turn on the circulator. Alternatively, you could pick up a 120vac thermostat and control the circulator directly. Note, your existing system may have an unsed set of contacts that could be used with a new thermostat and no new hardware other than the thermostat and rewiring.
So, in reality, there's no one standard way to do this, and you'd have to know more details about what you are starting with.
Just controlling the circulator may not fire the boiler, depending on how that is set up. You'd have to review what you have, and then see what is required to add a zone.