You can try "Jackroad" as recommended by Jeremy2101 which is located inside Nakano Broadway Mall. It is about 5 minutes walk away from JR Nakano station (not Nagano as Nagano is another prefecture). I have never been to Ginza but apart from Jackroad, there is another store "Kamekichi" which is worth to check out as well. You can google "Kame-Kichi" for the store's information.
This store is in the same building as Jackroad and on the same storey if I remember correctly. I bought my Omega Speedmaster Tokyo2020 Panda (worn but in very good condition) at a good price from Kame-Kichi.
Good luck exploring and enjoy your coming vacation in Tokyo! ;)