I have been to three auto parts stores so far with no luck but it does look like I have a lead on a used one.
I also took a picture inside the master cylinder threads and my original (not the one in the first picture in this discussion) master cylinder looks a little different than Ed's. It doesn't look like inverted flare. Maybe pipe thread? Which would make sense to me since the threads needs to make a tight leak free seal at this point unlike the female side of the missing fitting which would be inverted flare.
