I know that this is not the answer you are looking for, but I looked for a replacement rear sight also and could not find one. What I ended up doing was make a peep sight out of a swing swivel stud and tapping the cocking peace to make a poor man’s Rigby peep sight. Here is the link:

http://forums.accuratereloading.com/eve/forums/a/tpc/f/9411043/m/3871060032