WELCOME NZ!
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!!
Good Luck!!