I have a 5 zone boiler and all my thermostat only had 2 wires. I needed up using this.

24 Volt Transformer, C Wire Adapter Thermostats, Compatible with Ecobee, Nest and Honeywell Smart Wifi Thermostat, Ring Nest Hello Skybell August Doorbell (White) https://www.amazon.com/dp/B07MT5VWQN/ref=cm_sw_r_cp_api_glt_fabc_GN1SXZD9PV2F2A7MN9MS

This will feed the power to the C wire. Not an elegant solution but it works. It used it on mine a year ago

For the heat only ecobees, I've had good success with this: https://www.amazon.com/Transformer-Thermostats-Compatible-Honeywell-Thermostat/dp/B07MT5VWQN

The ecobee for the AC must use power from the AC wiring.