San never rode Moro.. so it must have been one of 
the wolf brothers.... 

Just finished watching the Movie and you are
correct!  Of the two Cels I saw with San riding
a wolf, the first was of San riding in the forest,
the second was from the scene San glares back at
Ashitaka before entering the forest (her face 
bloodied from attempting to suck the poison from
her mother).  I was extremely tempted to purchase
a Cel of San wearing the Mononoke Mask, arms
reaching out, dagger drawn ($1600), but
unfortunately it was early in my trip and I 
expected better things in Tokyo.  In hindsight,
I probably should have bought it.

All the Mononoke Cels were at Mandarake,
however the Osaka store had the nicest (4 or 5).
Shibuya and Nakano had 3 to 4 Cels each. Nakano
had a Great-looking Shishigami Cel ($1200)!

They had also started hyping Miyazaki's new film!
I couldn't translate the title, but the stills
looked fantastic!  It was hard to make out the
plot...but from the looks of it, it has strong
super-natural overtones!