I use a Blackboar 5-16x14 ferruleless shaft with a ebony 5-16x14 to 3-8x11 mid extension (made by Newsherifintown) on my Omen cue.
Just an idea to broaden your search. I believe Newsherifintown also could make you a ferruleless shaft for a 3-8x11 thread.
Good luck with your search.