For your first time, go to this shopping plaza, 
Nakano Broadway is more than enough for you to 
buy cels.  You can spend an entire day there 
looking at cels and other anime stuffs.  Well, 
depending which kind of cels that you get.  The 
majority of the popular cel stores in Tokyo are 
all located in this shopping plaza, it really 
saves you an extra trip to the other locations.  
It has famous cel stores like Anime World Star, 
Mandarake, Zero and Commit.  For more info, click 
on the link above.