NRT to Osaka upon arrival

Hello!

Im from Singapore and Im trying to get tickets online for the shinkansen from Tokyo to Osaka, right after I reach Narita airport. I want to get 3 tickets to and from Osaka and Tokyo.

I will reach Tokyo on the 30 Dec 2018 about 11am. I plan to go to Osaka on the same day and return back to Tokyo on the 2 Jan 2019.

I know the tickets can be brought in Tokyo but it has to be a day or so in advance, right?

I have already brought the air tickets and booked the hotels. Plus, I will be there during the peak period. So I have to buy the tickets online/outside of Japan. Where could I buy them?

Also, would a domestic flight or overnight bus be a better option?

Thank you for the help!