I have older HVAC unit (heat pump) and looking for the C wire to hook up to Smart Thermostat
You need a "Power Extender Kit (PEK) from Ecobee. I am surprised that the PEK wasn't included in your purchase of the smart thermostat. Mine was.

Visit their web site at 

https://support.ecobee.com/hc/en-us/articles/234707528-ecobee3-lite-with-PEK-for-HeatPumps

Their web site also has a "Live Support" ; or, you can call them at 1.877.932.6233.  They provide very good support service.

Sert