Hi Nikki,
   Your best bet is the Nakano Broadway Centre in 
Nakano. It is Mandarakes main store and houses a 
number of cel stores like Anime World Star.
To get there you will need to take a train to 
Shinjuku station (on JR Yamanote line) then 
switch to the line that goes to Nakano (Sorry 
can't remember name). I believe the Nakano stop 
is 3 stops from Shinjuku.
   At Nakano station go out through the North 
Exit. Cross the double pedestrian crossing in 
front of you and you will end up at the entrance 
to a very long enclosed mall tunnel.
   Walk all the way through the mall tunnel and 
you will end up at the Nakano Broadway Centre on 
the other side. The anime related and cel shops 
are level 2 to 4.
Hope this helps,
- X.
ps. Try a forum search of 'Tokyo' and you should 
pick up more threads with other places in Tokyo 
to shop at.