Colonial Drugs in Cambridge, MA might be able to help you. Last time I was in there they had Plisson brushes and they do ship. Their number is
(617) 864-2222.
Lets face it Plisson isn't exactly the chevy of brushes. Nobody really carries any large inventories . Most have a few here and a few there. I got mine from the shaving shop and they seemed to have about the biggest selection but you have to hit it just right. If you go for their HMW in a decent size thats alot of money for a brush.