The only duckbill chokes i have seen were made by the navy at NWC CHINA LAKE for the navy seals and other gov. agent. There may have been a small company that made some for police only. I have been looking at gun shows for about 10 years and never found them for sale.
when CHINA LAKE made them they made about 5000.
Post a question over on Subguns.com. They were being made by some guys in IL I believe. Ask for Chuck Madurski or Kevin Dockery. I recall them being involved with this and they may even be the manufacturers.