Can I run a C wire from Fan Relay
Every furnace that has an R wire also has a C wire.  Those are the two low-voltage terminals on the transformer. 

Generally wiring is in a functional triangle.  R goes from transformer to thermostat, W/G/Y and other circuits go from thermostat to relay, and C goes from relay back to transformer to complete the circuit.

You can grab C anywhere you find convenient.   Usually the thermostat wire goes close to the transformer or control board so most people grab it there.  But wherever!