There are 2 good stores I know of in New York City that sell used manga (such as Tezuka)
1. Book Off - East 41st Street btw Madison & Fifth
This store is the best. It's a used bookstore owned by a Japanese company. The second floor is all used manga. 80% of it is in Japanese, and 20% is in English. I check the English section every week. I have found excellent old stuff there. They have a ton of old Tezuka books in Japanese.
2. The Strand - 12th St & Broadway in Union Square
This is the best used bookstore in NYC. It is giant. On the second floor there is a small bookcase where you can find some good stuff.
Does anyone know of any others? Thanks.