GobulPower wrote: I'd suggest battling G rank Rathian over and over until you get one.
Thanks GobulPower, I'll do that
Yes, except G Rank Rathian drops Mantles, not wyvern stones. You'd either have to be incredibly lucky or a supreme hacker to get a Lrge Wyvern Stone out of that. You should do Plesioth, Qurupeco, Barroth, Ludroth, or Gigginox, really any monster that doesn't have it's own rare drop (e.g. Pallium, Mantle, DrgnGem, etc.)