My Disco hasn't used much in 6 months some owners have filled up using halfords stuff. But if it's free as part of the service plan then just as well getting the dealer to do it.
I have an MY16 2.0ed4 SE tech with the LR service pack. Adblue is free for the duration of the servicing. Just pop in to a dealer and they'll top it up FOC.
Another note i's if you run out and switch off your engine, you will not be able to restart without topping it up.