Chinese/Japanese Brush Painting Supplies

Try Daso (Aeon mall). They had fluid ink, Xuan (rice) paper, and only 1 size brush,  but all are not always in stock.
I got all my Chinese caligraphy and painting in Saigon.

Thanks Margot.  Daiso may have something.  I have actually been in a Daiso in Los Angeles, so I assume it is similar here.  I'll check it out.

Basic art materials can be found sometimes in one of the stalls that sell stationary in the Russian Market in Phnom Pen. I was up in Battambang recently, and spoke to an artist there about art materials. I was surprised to hear that he had to source all his stuff from Thailand. It might be an idea to phone one of the International Schools in Phnom Penh and enquire if they know of anywhere locally.

Thanks for the suggestion.  It turns out I have to return to the US for a little while, and can obtain exactly what I need there before returning.