I would have to agree with Tanaka-san here.
I went to Japan this summer, but fortunately it
wasn't for buying cels. I have aging family to
care for, and that was my primary reason for my
visit. 

However, I did take a break and visit some friends
in Tokyo.  I tried to do a bit of shopping for
cels there.  Akihabara is a huge disappointment
cel wise.  I got some nice doujinshi, used books 
and classic games, but for cels? Nothing.

I also went to Nakano. The selection was better, 
but I didn't have much time.  That sucked.  There
were oh so many more cel books I wanted to flip 
through but couldn't -_-

What I did see was rather mediocre, or the good 
stuff was over priced.  

The best cels I got the whole trip came from a 
friend of mine who works at Mad House and gave me
a stash of stuff rotting under his desk. 

Overall Advice:
If you are thinking of going to Japan for cels,
I would highly recommend against it.  If you can 
make it to a TBS event, I know TBS started taking
and selling cels from shows they produced at these
fairs, including the really good and the junk.
That's only good if you can make it to a TBS fair
of a show you like.....Doing THAT is another a 
horse of a different color.

I'm afraid you also can't waltz into any studio 
and announce you'd like to buy their cels. If you
do, you'd have to get it by the truck load, junk
cels and all.
I think some studios have stopped this practice,
so you may be SOL as far as going to studios go.

I would recommend you go to Japan for cultural 
purposes then make cel shopping something on the 
side.  There are so many other wonderful things
to do and see there, and I'd hate to see you waste
all that time hoping for cels and not getting a
chance to experience the culture.