Narita airport by Narita express

My flight leaves at midday. Is it adviseable to purchase a Narita express ticket before day of departure or would it be ok to buy it on the day at Tokyo station?

Thank you.

It's advisable to buy the NEX ticket if you have already decided what time you leave Tokyo, though NEXs are rarely booked out.

You might find a long queue at the ticket office or need some time to find it.

agreed, but note you don't have to buy it at Tokyo station if that's not convenient: you cab buy it at most major stations with ticket offices

you can also buy the tickets at most stations that have automatic ticket machines, usually they're located next to the station's main information/customer service office. There you can buy shinkansen and special train tickets / passes. You can also print out a receipt from the machine if you need to.