It looks like the last Keisei train to Nippori is at 10:41 pm, arriving at 11:54pm. There are Yamanote Line trains from Nippori to Shinagawa at 11:58, 00:04, 00:12 - every few minutes until 00:45.

You can't buy tickets outside the country so just get them from the Keisei station at NRT. I think your bigger question is whether you can clear customs at immigration in time to make that last train from NRT. If all goes well, you can. Otherwise, you might want to consider an airport hotel for that first night and make the trip in to Shinagawa in the morning.