Not that I'm any expert, but I'm not surprised to 
hear that you can't find a particular character.  
Why?  Well, I've heard the same story from several 
cel dealers and folks that visit Japan---there are 
hardcore Japanese collectors out there who will 
focus on a particular character, scene, or (with 
deep pockets) an entire series and suck everything 
up into a massive private collection.  I'm willing 
to bet that there are one or two houses in Japan 
that have binders full of just about every cel of 
the character you are looking for.....

The optimistic thought is that one day they may 
need money or may get bored and will let a few go 
fairly cheap, since they have an abundance of 
them.  ;-)

JP