You cannot find Israphel's castle in a minecraft world unless you build it or download a MCedit schematic or the yogscast map where the castle is. Unless you do one of those things there is no way...
Some people believe it is true while others don't but i can guarantee you he is gone now that Minecraft beta 1.8 he is gone Notch removed him. If you do see him take a snapshot of him and hide.