You can buy it at the normal ticket counter (as in Midorino Madoguchi) as well, but the problem tends to be that they don't always speak English well and also not everyone knows about them (although I hope they do at NRT).

At least the Travel Service Centre staff are guaranteed to know about them, making one's life easier. It keeps things a bit more simple and also avoids the confusion with the Keisei ticket counter if you turn left as you get to the train area at NRT T2.