I bought my Suica from a vending machine at Tokyo Station after returning from Hiroshima. (I went from NRT to Shinagawa, spent the night, Shinkansen to Hiroshima, took the long way back to Tokyo via Shikoku, all on a JR Pass.)

I don't recall whether you can buy a Suica at the JR Office at NRT. You won't need it to get into Tokyo if you're riding the N'EX or the Skyliner, in any case.