Well, I would think a 40 would be an easier size to get as opposed to the 30/35 sizes, purely because of popularity, however, I know at my store it's pretty rare to see such large bags - are you close to a store? The larger stores would be your best bet, I would say. Preices I can't help yo with as I'm not in the States ( assume that's where you are?)....won't be long and the other members will chime in - they know everything here!!
40cm do pop up in the NYC store on occasion, and they usually make it onto the floor because most people consider them too big. 45cm has to be pretty much SO'ed. Last time I saw a 40cm it was black togo with gold hardware and went for $8050. A 45cm would probably go for over $9000.