Try Steve Maxwell. He no longer offers his repro rail mounts for sale but he does still have some in stock. You might be able to sweet talk him into selling you just the spade arms.
Do you mean the metal strap that grips the spade arm?
Once upon a time someone was selling repros of those. If I remember right it was Cherie Willoughby, who seems to have dropped out of the vintage drum game about 15-20 years ago...so probably a dead end there.
I always upgrade the "phone dial" style rail uprights to the later ratchet style.