Buying JR tickets after JR Pass period finishes

Our family of 4 (3 adults & a child) are visiting Tokyo / Hakone for 8 days. We are landing in Narita and would be heading to Tokyo. We would also be leaving from Tokyo back to Narita. We were thinking of obtaining a 7-day JR pass. May I know on the 8th day, which is also the last day of our trip, how can we go about buying train tickets from Tokyo to Narita? Thank you.