Cels Forum

Kinokuniya will also do mail order if you call them, though
their shipping is a bit much.  
The only problem I've found with the Japanese cel books is 
that the covers are very flimsy and they don't offer any kind
of support for the cels.  They also look cheep in my opinion.
I do recommend your second suggestion of US imported Itoya type
portfolios.  Though I've only seen black covers, they are much
nicer looking and they protect cels better. 

Metaphor